Optical structures providing dichroic effects

ABSTRACT

A document, product, or package, such as a banknote, passport or the like comprises structures having dichroic effects that change color with viewing angle in both transmission and reflection. Such structures can be useful as security features that counter the ability to effectively use counterfeit documents, products, packages, etc.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 16/054,898 filed Aug. 3, 2018, which claims the benefit of priority of U.S. Provisional Application No. 62/568,711 filed on Oct. 5, 2017. The entirety of each application referenced in this paragraph is expressly incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED R&D

This invention was made with government support under Contract No. TEPS 16-34769 awarded by the Bureau of Engraving and Printing. The government has certain rights in the invention.

TECHNICAL FIELD

The present application generally relates to thin interference optical structures, films, coatings and pigments for producing color in both reflection mode and transmission mode. More specifically, these structures, films, coatings, and pigments exhibit large color shifting properties with changes in both reflection and transmission potentially with a change in the angle of incidence or the viewing angle.

DESCRIPTION OF THE RELATED TECHNOLOGY

Color shifting features can be used as a security device (for example, on a banknote) to prevent counterfeiting. The color shifting effect produced by the color shifting materials can be easy for the common person to observe. The color shifting effect produced by the color shifting features, however, can be impractical to recreate using counterfeit copies produced by color copiers, printers and/or photographic equipment. Color copiers, printers and/or photographic equipment use pigments based on dyes having absorption and as such the printed colors can be insensitive to a change in the viewing angle. Therefore, the difference between an authentic document comprising color shifting features and a fake one can be detected by tilting the document to observe if there is a color shift. Some color shifting features that are available are opaque and exhibit a color shift for reflection mode. Additionally, counterfeiters have developed sophisticated methods that compromise the effectiveness the existing reflective color shifting features as counterfeit protection. Thus, with respect to security devices, a new security feature that is difficult to counterfeit and can be readily incorporated into an item such as a banknote is desirable.

SUMMARY

This application discloses and contemplates a wide variety of structures including some at least partially transmissive optical structures. Advantageously, variations of such at least partially transmissive optical structures can present a color shift in both reflection mode and transmission mode with respect to viewing angle. Also, variations of such at least partially transmissive optical structures can be integrated with documents (e.g., a banknote), packaging as well as potential other items to, for example, enhance security and/or prevent counterfeiting. Although such features described herein can be used in security applications such as reducing the incidence of counterfeiting, alternatively or in addition, such feature could be used for providing an aesthetic effect or for other reasons.

This application contemplates documents, products, and packaging with features (e.g., security features) that provides an optical effect of changing color with angle of observation in both reflection and transmission. The color shift with respect to viewing angle in both reflection and transmission can be achieved by incorporating the at least partially transmissive optical structures in the document, product, packaging etc., as a security feature. The at least partially transmissive optical structures can be a dichroic structure. The at least partially transmissive optical structures can be in the form of a thin film coating on a flexible support or base layer such as a sheet, web or carrier. In some embodiments, the at least partially transmissive optical structures comprise a pigment. In some cases, an assembly of particles comprising the at least partially transmissive optical structures can be included in a medium and form, for example an ink. The optical effect from the assembly of particles can provide a color shift in reflection and transmission. The color in transmission may be the compliment color of the color perceived in reflection mode. In some such embodiments, each particle can comprise the same structure or similar structures.

Some implementations of the at least partially transmissive optical structures contemplated herein can comprise at least two metal layers that sandwich at least one transparent layer between the at least two metal layers. The at least one transparent layer sandwiched between the at least two metal layers can have a refractive index that is greater than, less than or equal to 1.65. The at least partially transmissive optical structures contemplated herein can further comprise transparent layers on the other side of the at least two metal layers. The transparent layers on the side of the at least two metal layers opposite the side facing the sandwiched at least one transparent layer can have a refractive index greater than or equal to 1.65. The at least two metal layers can comprise metals that have a ratio of their real (n) and imaginary (k) refractive index less than 1.0. Accordingly, the metals of the at least two metal layers can have the ratio n/k<1. Without any loss of generality, the real part n is the refractive index and indicates the phase velocity, while the imaginary part k is called the extinction coefficient and can relate to absorption. The at least two metal layers can comprise silver, silver alloys, aluminum, gold, as well as other metals or materials or combination thereof.

Various optical structures contemplated in this application can provide color shift when viewed in reflection and transmission mode as a function of viewing angle. Hence these structures can be incorporated as security features for documents such as banknotes or other documents to verify authenticity of the documents. Structures contemplated in this application can be configured to be used as a security thread, as a laminate, as a hot stamp, as a window patch or as pigment. The laminate comprising a substrate (e.g., PET), the dichroic thin film and the protective UV cured resin can be adhered as a unit to the banknote with an adhesive. Structures contemplated in this application can be configured to be used in a printing ink. Non-shifting transparent dyes or pigments can be incorporated with the optical structures contemplated in this application to obtain new colors when viewed in reflection and transmission mode. It is further contemplated that the two or more at least partially transmissive optical structures can be disposed over each other (e.g., printed or laminated over each other) to produce unique color effects. The at least partially transmissive optical structures contemplated herein can be configured or arranged to form, include or otherwise display text, symbols, numbers or figures that appear and/or disappear in reflection or transmission as the viewing angle of the security device is changed. In other configurations, the figures, images, numbers, pictures or symbols can be viewed at substantially all angles in transmission. For example, if the figures, images, numbers, pictures or symbols are printed in black, then they can be viewed at substantially all angles in transmission. In some cases, for example text, numbers, pictures or symbols can be underprinted and/or overprinted under and/or over the at least partially transmissive optical structures using existing printing technologies.

The at least partially transmissive optical structures can be included in or on or configured as a film, a foil, a coating, a pigment or an ink. When configured as a pigment, in some implementations, the pigment can be encapsulated with a protective layer. The protective layer can comprise SiO₂. The protective layer can comprise a solution prepared using a sol-gel technology such as, for example, acid or based catalyzed tetraethylorthosilicate (TEOS) reactions for increased durability. In some cases, the protective layer can further comprise silica spheres having same or different sizes. A silane coupling agent can be bonded with the protective layer comprising silica (SiO₂). The silane coupling agent can be bonded to a resin, ink or paint vehicle. The resin, ink or paint vehicle can comprise a material, such as, for example, acrylic melamine, urethanes, polyesters, vinyl resins, acrylates, methacrylate, ABS resins, epoxies, styrenes and formulations based on alkyd resins and combinations or mixtures thereof. In some implementations, the at least partially transmissive optical structures can be encapsulated, for example, with an encapsulating layer having a refractive index that matches or closely matches the refractive index of the article to which it is applied. In certain implementations, the encapsulating layer can comprise a rough surface so that particles will not tend to stick together or stick to print rollers. The encapsulating layer can comprise a UV curing polymer.

These and other aspects of the at least partially transmissive optical structures will be apparent from their accompanying drawings and this specification.

The at least partially transmissive optical structures disclosed herein can be used for security features included in documents, products, packages, etc., in particular, as security threads in bank notes or as a laminated strip, or as a patch or as a window. Other items such as passports, ID cards, chip cards, credit cards, stock certificates or other investment securities, vouchers, admission tickets as well as commercial packages that protect items of value such as CD's, medicinal drugs, car and aircraft parts, etc. may also be protected against counterfeiting using the concepts and embodiments described herein. Furthermore, the at least partially transmissive optical structures disclosed herein can also be used for non-security applications.

Although some of the optical structures discussed herein can provide color shift with viewing angle, optical structures that do not exhibit color shift with change in viewing angle or produce very little color shift with change in viewing angle are also contemplated.

The systems, methods and devices disclosed herein each have several innovative aspects, no single one of which is solely responsible for the desirable attributes disclosed herein. A variety of example systems and methods are provided below.

Example 1: An optical structure comprising:

a first transparent dielectric layer having a refractive index greater than or equal to 1.65;

a first metal layer disposed over the first transparent dielectric layer, the first metal layer having a first refractive index, wherein a ratio of the real part (n) of the first refractive index to the imaginary part (k) of the first refractive index (k) is greater than or equal to 0.01 and less than or equal to 0.5;

a second transparent dielectric layer disposed over the first metal layer;

a second metal layer disposed over the second transparent dielectric layer, the second metal layer having a second refractive index, wherein a ratio of the real part (n) of the second refractive index to the imaginary part (k) of the second refractive index is greater than or equal to 0.01 and less than or equal to 0.5; and a third transparent dielectric layer disposed over the second metal layer, the third transparent dielectric layer having a refractive index greater than or equal to 1.65.

Example 2: The optical structure of Example 1, wherein the second transparent dielectric layer has a refractive index less than 1.65.

Example 3: The optical structure of any of Examples 1-2, wherein the second transparent dielectric layer has a refractive index greater than or equal to 1.65.

Example 4: The optical structure of any of Examples 1-3, having a transmission peak comprising:

a maximum transmittance greater than 50%; and

a spectral bandwidth defined by a full width of the transmission peak at 50% of the maximum transmittance,

wherein the maximum transmittance is at least 50%, and

wherein the spectral bandwidth of the transmission peak is greater than 2 nm.

Example 5: The optical structure of Example 4, wherein the spectral bandwidth of the transmission peak is greater than or equal to about 10 nm and less than or equal to about 200 nm.

Example 6: The optical structure of any of Examples 4-5, wherein the maximum transmittance is at a wavelength between about 400 nm and about 700 nm.

Example 7: The optical structure of any of Examples 4-6, further comprising a reflection peak comprising:

a maximum reflectance; and

a spectral bandwidth defined by a full width of the reflection peak at 50% of the maximum reflectance,

wherein the maximum reflectance is at least 50%, and

wherein the spectral bandwidth of the reflection peak is greater than 2 nm.

Example 8: The optical structure of Example 7, wherein the spectral bandwidth of the reflection peak is greater than or equal to about 10 nm and less than or equal to about 200 nm.

Example 9: The optical structure of any of Examples 7-8, wherein the maximum reflectance is at a wavelength between about 400 nm and about 700 nm.

Example 10: The optical structure of any of Examples 7-9, wherein the maximum transmittance is at a first wavelength, and wherein the maximum reflectance is at a second wavelength different from the first wavelength.

Example 11: The optical structure of any of Examples 1-10, configured to display a first color when viewed by an average human eye along a direction normal to a surface of the optical structure in reflection mode and a second color different from the first color when viewed by an average human eye along a direction normal to a surface of the optical structure in transmission mode.

Example 12: The optical structure of Example 11, wherein the first color shifts to a third color when viewed by an average human eye along a direction at an angle away from the normal to the surface of the optical structure in reflection mode.

Example 13: The optical structure of any of Examples 11-12, wherein the second color shifts to a fourth color when viewed by an average human eye along a direction at an angle away from the normal to the surface of the optical structure in transmission mode.

Example 14: The optical structure of any of Examples 1-13, wherein the first or the second metal layer has a thickness greater than or equal to about 5 nm and less than or equal to about 35 nm.

Example 15: The optical structure of any of Examples 1-14, wherein the second transparent dielectric layer has a thickness greater than or equal to about 100 nm and less than or equal to about 2 microns.

Example 16: The optical structure of any of Examples 1-15, wherein first or the third transparent dielectric layer has a thickness greater than or equal to about 100 nm and less than or equal to about 500 nm.

Example 17: The optical structure of any of Examples 1-16, further comprising an encapsulating layer comprising silica.

Example 18: The optical structure of Example 17, wherein the silica is bonded to a silane coupling agent.

Example 19: The optical structure of Example 18, wherein the silane coupling agent is configured to bind to an ink or paint medium.

Example 20: The optical structure of any of Examples 1-19, wherein the first or the second metal layer comprises at least one of aluminum, silver, gold, silver alloy, or gold alloy.

Example 21: The optical structure of any of Examples 1-20, wherein the second transparent dielectric layer comprises a material having a refractive index less than 1.65, greater than 1.65 or equal to 1.65.

Example 22: The optical structure of any of Examples 1-21, wherein the second transparent dielectric layer comprises at least one of SiO2, MgF2 or a polymer.

Example 23: The optical structure of any of Examples 1-22, wherein the first or the third transparent dielectric layer comprises at least one of zinc oxide (ZnO), zinc sulfide (ZnS), zirconium dioxide (ZrO2), titanium dioxide (TiO2), tantalum pentoxide (Ta2O5), ceric oxide (CeO2), ytterium oxide (Y2O3), indium oxide (In2O3), tin oxide (SnO2), indium tin oxide (ITO), tungsten trioxide (WO3), or combinations thereof.

Example 24: The optical structure of any of Examples 1-23, wherein the first or the second metal layer has a thickness greater than or equal to about 5 nm, or less than or equal to about 35 nm.

Example 25: The optical structure of any of Examples 1-24, wherein the second transparent dielectric layer has a thickness greater than or equal to about 100 nm, or less than or equal to about 700 nm.

Example 26: The optical structure of any of Examples 1-25, wherein the first or the third transparent dielectric layer has a thickness greater than or equal to about 100 nm, or less than or equal to about 500 nm.

Example 27: The optical structure of any of Examples 1-26, configured as a pigment, a paint or an ink.

Example 28: The optical structure of any of Examples 1-27, further comprising a base layer configured to support the first dielectric layer, wherein the optical structure is configured as film.

Example 29: The optical structure of Example 28, wherein the base layer is flexible.

Example 30: The optical structure of any of Examples 28-29, wherein the base layer comprises a polymer.

Example 31: The optical structure of any of Examples 28-30, wherein the film is surrounded by a protective barrier.

Example 32: The optical structure of Example 31, wherein the protective barrier comprises a UV curable resin.

Example 33: The optical structure of any of Examples 1-32, further comprising an encapsulating layer, wherein the optical structure is configured as a pigment, a paint or an ink.

Example 34: The optical structure of Example 33, wherein the encapsulating layer comprises silicon dioxide (SiO2).

Example 35: The optical structure of any of Examples 33-34, further comprising a plurality of silica spheres embedded in the encapsulating layer.

Example 36: The optical structure of Example 35, wherein some of the plurality of silica spheres have a size different from a size of some other of the plurality of silica spheres.

Example 37: The optical structure of any of Examples 33-36, wherein the encapsulating layer is chemically attached to a silane coupling agent, the silane coupling agent comprising a reactive group that is configured to chemically bond with an ink or a paint medium.

Example 38: The optical structure of Example 37, wherein the ink or the paint medium comprises a material selected from the group consisting of acrylic melamine, urethanes, polyesters, vinyl resins, acrylates, methacrylate, ABS resins, epoxies, styrenes and formulations based on alkyd resins and mixtures thereof.

Example 39: The optical structure of any of Examples 37-38, wherein the ink or the paint medium comprises a resin or a polymer.

Example 40: A banknote or a document comprising the optical structure of any of Examples 1-39.

Example 41: The banknote or document of Example 40, wherein the optical structure is configured as laminate that is attached to the banknote or document.

Example 42: The banknote or document of Example 40, wherein the optical structure is configured as a security thread that is inserted in the banknote or document.

Example 43: The banknote or document of Example 40, wherein the optical structure is configured as a label that is attached to the banknote or document.

Example 44: The banknote or document of Example 40, further comprising a window, wherein the optical structure is incorporated in the window.

Example 45: A document having a security feature comprising:

a main body of the document; and

an optical structure comprising:

a first transparent dielectric layer having a refractive index greater than or equal to 1.65;

a first metal layer disposed over the first transparent dielectric layer, the first metal layer having a first refractive index, wherein a ratio of the real part (n) of the first refractive index to the imaginary part (k) of the first refractive index (k) is greater than or equal to 0.01 and less than or equal to 0.5;

a second transparent dielectric layer disposed over the first metal layer;

a second metal layer disposed over the second transparent dielectric layer, the second metal layer having a second refractive index, wherein a ratio of the real part (n) of the second refractive index to the imaginary part (k) of the second refractive index is greater than or equal to 0.01 and less than or equal to 0.5; and

a third transparent dielectric layer having a refractive index greater than or equal to 1.65 disposed over the second metal layer,

wherein the optical structure is configured to display a first color in reflection mode and display a second color different from the first color in transmission mode.

Example 46: The security document of Example 45, further comprising a second optical structure comprising:

a fourth transparent dielectric layer having a refractive index greater than or equal to 1.65;

a third metal layer disposed over the fourth transparent dielectric layer, the third metal layer having a third refractive index, wherein a ratio of the real part (n) of the third refractive index to the imaginary part (k) of the third refractive index (k) is greater than or equal to 0.01 and less than or equal to 0.5;

a fifth transparent dielectric layer disposed over the third metal layer;

a fourth metal layer disposed over the fifth transparent dielectric layer, the fourth metal layer having a fourth refractive index, wherein a ratio of the real part (n) of the fourth refractive index to the imaginary part (k) of the fourth refractive index is greater than or equal to 0.005 and less than or equal to 0.5; and

a sixth transparent dielectric layer having a refractive index greater than or equal to 1.65 disposed over the fourth metal layer,

wherein the second optical structure is configured to display a third color in reflection mode different from the first and the second color and display a fourth color different from the first, second and the third color in transmission mode.

Example 47: The security document of Example 46, wherein the optical structure or the second optical structure is configured as a film attached to the main body of the document.

Example 48: The security document of any of Examples 46-47, wherein the optical structure or the second optical structure is configured as a thread inserted into the main body of the document.

Example 49: The security document of any of Examples 46-48, wherein the optical structure or the second optical structure is configured as a laminate disposed over the main body of the document.

Example 50: The security document of any of Examples 46-49, wherein the optical structure or the second optical structure is configured as an ink, a dye, or a paint contacting the main body of the document.

Example 51: The security document of any of Examples 46-50, further comprising a first window comprising the optical structure and a second window comprising the second optical structure.

Example 52: The security document of any of Examples 46-51, wherein the optical structure is configured as a dichroic ink, a dichroic pigment or a dichroic paint that is configured to produce a first color at a first viewing angle and a second color at a second viewing angle.

Example 53: The security document of any of Examples 46-52, wherein the document is printed with the dichroic ink, the dichroic pigment or the dichroic paint.

Example 54: The security document of Example 53, wherein the dichroic ink, the dichroic pigment or the dichroic paint is disposed over, under or mixed with a non-dichroic ink, pigment, or paint that is configured to produce the first color at the first and the second viewing angles.

Example 55: The security document of Example 54, wherein the non-dichroic, ink pigment or paint forms a text, an image, a number or a symbol.

Example 56: The security document of Example 55, wherein the text, the image, the number or the symbol is invisible at the first viewing angle and visible at the second viewing angle.

Example 57: A method of manufacturing a security feature configured to produce a first color in reflection mode and a second color in transmission mode, the method comprising:

providing a base layer; and

disposing an optical structure on the base layer, the optical structure comprising:

-   -   a first transparent dielectric layer on the base layer, the         first transparent dielectric layer having a refractive index         greater than or equal to 1.65;     -   a first metal layer disposed over the first transparent         dielectric layer, the first metal layer having a first         refractive index, wherein a ratio of the real part (n) of the         first refractive index to the imaginary part (k) of the first         refractive index (k) is greater than or equal to 0.01 and less         than or equal to 0.5;     -   a second transparent dielectric layer disposed over the first         metal layer;     -   a second metal layer disposed over the second transparent         dielectric layer, the second metal layer having a second         refractive index, wherein a ratio of the real part (n) of the         second refractive index to the imaginary part (k) of the second         refractive index is greater than or equal to 0.01 and less than         or equal to 0.5; and     -   a third transparent dielectric layer disposed over the second         metal layer, the third dielectric layer having a refractive         index greater than or equal to 1.65.

Example 58: The method of Example 57, wherein disposing the optical structure on the base layer comprises:

coating the first transparent dielectric layer on the base layer;

depositing the first metal layer on the first transparent dielectric layer;

disposing the second transparent dielectric layer on the first metal layer;

depositing the second metal layer on the second transparent dielectric layer; and

disposing the third transparent dielectric layer on the second metal layer.

Example 59: The method of any of Examples 57-58, further comprising:

cutting a strip of the base layer with the optical structure; and

coating the strip with a UV curable polymer to obtain a security thread.

Example 60: The method of any of Examples 57-58, further comprising:

removing the optical structure from the base layer;

fragmenting optical structure into platelets having an area that is between five times and about ten times the thickness of the optical structure;

encapsulating the platelet in an encapsulation layer comprising a plurality of silica spheres;

attaching a silane coupling agent to the encapsulating layer; and

mixing the platelets with an ink or a paint medium to obtain a dichroic ink or paint.

Example 61: The method of any of Examples 57-60, wherein the base layer is flexible.

Example 62: The method of any of Examples 57-61, wherein the base layer comprise web.

Example 63: An optical structure comprising:

a substrate;

a first optical structure over the substrate; and

a second optical structure over the substrate, the first optical structure and the second optical structure at least partially overlapping,

wherein the each of the first and the second optical structure comprises:

a first transparent dielectric layer having a refractive index greater than or equal to 1.65;

a first metal layer disposed over the first transparent dielectric layer, the first metal layer having a first refractive index, wherein a ratio of the real part (n) of the first refractive index to the imaginary part (k) of the first refractive index (k) is greater than or equal to 0.01 and less than or equal to 0.5;

a second transparent dielectric layer disposed over the first metal layer;

a second metal layer disposed over the second transparent dielectric layer, the second metal layer having a second refractive index, wherein a ratio of the real part (n) of the second refractive index to the imaginary part (k) of the second refractive index is greater than or equal to 0.01 and less than or equal to 0.5; and

a third transparent dielectric layer disposed over the second metal layer, the third transparent dielectric layer having a refractive index greater than or equal to 1.65,

wherein a thickness of the various layers of the first optical structure is configured to reflect a first color and transmit a second color different from the first color, and

wherein a thickness of the various layers of the second optical structure is configured to reflect a third color different from the first color and transmit a fourth color different from the first, the second or the third color.

Example 64: The optical structure of Example 63, wherein the first and the second optical structures are completely overlapping.

Example 65: The optical structure of any of Examples 63-64, wherein the first and the second optical structures are configured as films.

Example 66: The optical structure of any of Examples 63-65, wherein the first and the second optical structures are configured as pigments.

Example 67: The optical structure of any of Examples 63-66, wherein the first and the second optical structures are configured as laminates.

Example 68: The optical structure of any of Examples 63-67, wherein the first and the second optical structures are configured as security threads.

Example 69: A document having a security feature comprising:

a main body of the document; and

a pigment disposed on the main body, the pigment comprising:

an optical structure comprising:

a first metal layer disposed over the first transparent dielectric layer, the first metal layer having a first refractive index, wherein a ratio of the real part (n) of the first refractive index to the imaginary part (k) of the first refractive index (k) is greater than or equal to 0.01 and less than or equal to 0.5;

a transparent dielectric layer disposed over the first metal layer; and

a second metal layer disposed over the transparent dielectric layer, the second metal layer having a second refractive index, wherein a ratio of the real part (n) of the second refractive index to the imaginary part (k) of the second refractive index is greater than or equal to 0.01 and less than or equal to 0.5; and an encapsulation layer encapsulating the optical structure.

Example 70: The document of Example 69, wherein the encapsulation layer comprises silica.

Example 71: The document of any of Examples 69-70, wherein the pigment produces a first color at a first viewing angle and a second color different from the first color at a second viewing angle.

Example 72: The document of any of Examples 69-71, wherein the pigment comprises a resin configured to chemically attach to the encapsulation layer.

Example 73: The document of any of Examples 69-72, wherein the optical structure has a thickness, and wherein a length or a width of the optical structure is at least 5 times the thickness.

Example 74: An optical structure comprising:

a dielectric region having an outer surface enclosing a volume of a dielectric material; and

a partially optically transmissive metal layer surrounding the outer surface of the dielectric region,

wherein a thickness of the optical structure has a value between about 100 nm and about 2 micron,

wherein a lateral dimension of the optical structure is between about 1 micron and about 20 micron, and

wherein the optical structure is configured to display a first color in a reflection mode and display a second color different from the first color in transmission mode.

Example 75: The optical structure of Example 74, further comprising a second dielectric region comprising one or more dielectric materials having a refractive index greater than about 1.65, the second dielectric region surrounding the partially optically transmissive metal layer.

Example 76: The optical structure of any of Examples 74-75, wherein the partially optically transmissive metal layer covers at least 80% of the outer surface of the dielectric region.

Example 77: The optical structure of any of Examples 74-76, wherein the partially optically transmissive metal layer covers at least 90% of the outer surface of the dielectric region.

Example 78: The optical structure of any of Examples 74-77, wherein the partially optically transmissive metal layer covers 100% of the outer surface of the dielectric region.

Example 79: The optical structure of any of Examples 74-78, wherein the dielectric region is spherical, ellipsoidal or round.

Example 80: The optical structure of any of Examples 74-79, wherein the dielectric region is a cube or a rectangular cuboid.

Example 81: The optical structure of any of Examples 74-80, wherein the dielectric region comprises a particle.

Example 82: The optical structure of any of Examples 74-81, wherein the partially optically transmissive metal layer comprises silver.

Example 83: The optical structure of any of Examples 74-82, wherein the partially optically transmissive metal layer has a thickness between about 3 nm and about 40 nm.

Example 84: The optical structure of any of Examples 74-83, wherein the dielectric region comprises silicon dioxide or titanium dioxide.

Example 85: The optical structure of any of Examples 75-84, wherein the second dielectric layer comprises a material having a refractive index greater than about 1.65.

Example 86: The optical structure of any of Examples 75-85, wherein the second dielectric layer comprises titanium dioxide.

Example 87: The optical structure of any of Examples 75-86, wherein the second dielectric layer covers at least 80% of the outer surface of the partially optically transmissive metal layer.

Example 88: The optical structure of any of Examples 75-87, wherein the second dielectric layer covers at least 90% of the outer surface of the partially optically transmissive metal layer.

Example 89: The optical structure of any of Examples 75-88, wherein the second dielectric layer covers at least 95% of the outer surface of the partially optically transmissive metal layer.

Example 90: The optical structure of any of Examples 75-89, wherein the second dielectric layer covers 100% of the outer surface of the partially optically transmissive metal layer.

Example 91: The optical structure of any of Examples 74-90, wherein said dielectric region comprises SiO₂.

Example 92: The optical structure of any of Examples 74-91, wherein said dielectric region comprises TiO₂.

Example 93: The optical structure of any of Examples 74-92, wherein said dielectric region comprises borosilicate with a high refractive index metal oxide layer thereon.

Example 94: The optical structure of any of Examples 74-93, wherein said dielectric region comprises borosilicate with TiO₂ thereon.

Example 95: The optical structure of any of Examples 74-94, wherein said dielectric region comprises borosilicate with SiO₂ thereon.

Example 96: The optical structure of any of Examples 74-95, included in a security thread or security ink.

Example 97: The optical structure of any of Examples 74-95, included in a film, a thread, a foil, or a laminate.

Example 98: The optical structure of any of Examples 74-95, included in a flexible film having a flexible base.

Example 99: The optical structure of any of Examples 74-95, included in a pigment, a paint or an ink.

Example 100: A security document comprising the optical structure of any of Examples 74-99.

Example 101: A security document comprising the optical structure of any of Examples 74-100, wherein the first color and second color are complementary colors.

Example 102: A method of manufacturing a dichroic ink or paint configured to produce a first color in reflection mode and a second color in transmission mode, the method comprising:

providing a base layer; and

disposing an optical structure on the base layer, the optical structure comprising:

a first metal layer disposed on the base layer, the first metal layer having a first refractive index, wherein a ratio of the real part (n) of the first refractive index to the imaginary part (k) of the first refractive index (k) is greater than or equal to 0/01 and less than or equal to 0.5;

a first transparent dielectric layer disposed over the first metal layer; and

a second metal layer disposed over the first transparent dielectric layer, the second metal layer having a second refractive index, wherein a ratio of the real part (n) of the second refractive index to the imaginary part (k) of the second refractive index is greater than or equal to 0.01 and less than or equal to 0.5.

Example 103: The method of Example 102, further comprising:

removing the optical structure from the base layer;

fragmenting optical structure into platelets having an area that is between five times and about ten times the thickness of the optical structure; and

dispersing the platelets in an ink medium or a paint medium to obtain a dichroic ink or paint.

Example 104: The method of Example 103, further comprising encapsulating an individual platelet in an encapsulation layer comprising a plurality of silica spheres.

Example 105: The method of Example 104, further comprising attaching a silane coupling agent to the encapsulating layer.

Example 106: The method of any of Examples 102-105, wherein the optical structure further comprises:

a second transparent dielectric layer between the base layer and the first metal layer, the second transparent dielectric layer having a refractive index greater than or equal to 1.65; and

a third transparent dielectric layer disposed over the second metal layer, the third dielectric layer having a refractive index greater than or equal to 1.65.

Example 107: A dichroic ink or paint configured to produce a first color in reflection mode and a second color in transmission mode, the dichroic ink or paint comprising:

a base layer; and

an optical structure on the base layer, the optical structure comprising:

a first metal layer disposed on the base layer, the first metal layer having a first refractive index, wherein a ratio of the real part (n) of the first refractive index to the imaginary part (k) of the first refractive index (k) is greater than or equal to 0.01 and less than or equal to 0.5;

a first transparent dielectric layer disposed over the first metal layer; and

a second metal layer disposed over the first transparent dielectric layer, the second metal layer having a second refractive index, wherein a ratio of the real part (n) of the second refractive index to the imaginary part (k) of the second refractive index is greater than or equal to 0.01 and less than or equal to 0.5.

Example 108: The dichroic ink or paint of Example 107, wherein the optical structure further comprises:

a second transparent dielectric layer between the base layer and the first metal layer, the second transparent dielectric layer having a refractive index greater than or equal to 1.65; and

a third transparent dielectric layer disposed over the second metal layer, the third dielectric layer having a refractive index greater than or equal to 1.65.

Example 109: The dichroic ink or paint of any of Examples 107-108, further comprising an ink medium or a paint medium comprising the optical structure,

wherein the optical structure has a thickness between 100 nm and 2 micron, and wherein a lateral dimension of the optical structure is between 1 micron and 20 micron.

Example 110: An optical structure comprising:

a dielectric region having an outer surface enclosing a volume of a dielectric material; and

a partially optically transmissive metal layer surrounding the outer surface of the dielectric region,

wherein a thickness of the optical structure has a value between about 100 nm and about 2 micron,

wherein a lateral dimension of the optical structure is between about 100 nm and about 20 micron, and

wherein the optical structure is configured to display a first color in a reflection mode and display a second color different from the first color in transmission mode.

Example 111: The optical structure of Example 110, further comprising a second dielectric region comprising one or more dielectric materials having a refractive index greater than about 1.65, the second dielectric region surrounding the partially optically transmissive metal layer.

Example 112: The optical structure of any of Examples 110-111, wherein the partially optically transmissive metal layer covers at least 80% of the outer surface of the dielectric region.

Example 113: The optical structure of any of Examples 110-112, wherein the partially optically transmissive metal layer covers at least 90% of the outer surface of the dielectric region.

Example 114: The optical structure of any of Examples 110-113, wherein the partially optically transmissive metal layer covers 100% of the outer surface of the dielectric region.

Example 115: The optical structure of any of Examples 110-114, wherein the dielectric region is spherical, ellipsoidal or round.

Example 116: The optical structure of any of Examples 110-115, wherein the dielectric region is a cube or a rectangular cuboid.

Example 117: The optical structure of any of Examples 110-116, wherein the dielectric region comprises a particle.

Example 118: The optical structure of any of Examples 110-117, wherein the partially optically transmissive metal layer comprises silver.

Example 119: The optical structure of any of Examples 110-118, wherein the partially optically transmissive metal layer has a thickness between about 3 nm and about 40 nm.

Example 120: The optical structure of any of Examples 110-119, wherein the dielectric region comprises silicon dioxide or titanium dioxide.

Example 121: The optical structure of any of Examples 111-120, wherein the second dielectric layer comprises a material having a refractive index greater than about 1.65.

Example 122: The optical structure of any of Examples 111-121, wherein the second dielectric layer comprises titanium dioxide.

Example 123: The optical structure of any of Examples 111-122, wherein the second dielectric layer covers at least 80% of the outer surface of the partially optically transmissive metal layer.

Example 124: The optical structure of any of Examples 111-123, wherein the second dielectric layer covers at least 90% of the outer surface of the partially optically transmissive metal layer.

Example 125: The optical structure of any of Examples 111-124, wherein the second dielectric layer covers at least 95% of the outer surface of the partially optically transmissive metal layer.

Example 126: The optical structure of any of Examples 111-125, wherein the second dielectric layer covers 100% of the outer surface of the partially optically transmissive metal layer.

Example 127: The optical structure of any of Examples 110-126, wherein said dielectric region comprises SiO₂.

Example 128: The optical structure of any of Examples 110-127, wherein said dielectric region comprises TiO₂.

Example 129: The optical structure of any of Examples 110-128, wherein said dielectric region comprises borosilicate with a high refractive index metal oxide layer thereon.

Example 130: The optical structure of any of Examples 110-129, wherein said dielectric region comprises borosilicate with TiO₂ thereon.

Example 131: The optical structure of any of Examples 110-130, wherein said dielectric region comprises borosilicate with SiO₂ thereon.

Example 132: The optical structure of any of Examples 110-131, included in a security thread or security ink.

Example 133: The optical structure of any of Examples 110-132, included in a thread, a foil, or a laminate.

Example 134: The optical structure of any of Examples 110-133, included in a flexible film having a flexible base.

Example 135: The optical structure of any of Examples 110-134, included in a pigment, a paint or an ink.

Example 136: A security document comprising the optical structure of any of Examples 110-135.

Example 137: A security document comprising the optical structure of any of Examples 110-136, wherein the first color and second color are complementary colors.

Example 138: The optical structure of any of Examples 1-26, configured as a foil, a thread or a laminate.

Example 139: The optical structure of any of Examples 110-112, wherein the partially optically transmissive metal layer covers at least 95% of the outer surface of the dielectric region.

Example 140: The optical structure of any of Examples 74-76, wherein the partially optically transmissive metal layer covers at least 95% of the outer surface of the dielectric region.

Example 141: The security document of Example 55, wherein the text, the image, the number or the symbol is invisible at the second viewing angle and visible at the first viewing angle.

Example 142: The method of Example 58, wherein disposing the second transparent dielectric layer on the first metal layer comprises depositing the second transparent dielectric layer on the first metal layer.

Example 143: The method of Example 58, wherein disposing the third transparent dielectric layer on the second metal layer comprises depositing the third transparent dielectric layer on the second metal layer.

Example 144: The method of Examples 57 or 58, further comprising:

removing the optical structure from the base layer;

fragmenting optical structure into platelets having an area that is between five times and about ten times the thickness of the optical structure;

attaching a silane coupling agent to the optical structure; and

mixing the platelets with an ink or a paint medium to obtain a dichroic ink or paint.

Example 145: The method of Example 144, further comprising: encapsulating the platelet in an encapsulation layer; and attaching the silane coupling agent to the encapsulation layer.

Example 146: The method of Example 58, further comprising depositing the first metal layer on the first transparent dielectric layer using an electroless method.

Example 147: The method of Example 58, further comprising depositing the second metal layer on the second transparent dielectric layer using an electroless method.

Example 148: A pigment comprising:

an optical structure comprising:

-   -   a first metal layer disposed over the first transparent         dielectric layer, the first metal layer having a first         refractive index, wherein a ratio of the real part (n) of the         first refractive index to the imaginary part (k) of the first         refractive index (k) is greater than or equal to 0.01 and less         than or equal to 0.5;     -   a transparent dielectric layer disposed over the first metal         layer; and     -   a second metal layer disposed over the transparent dielectric         layer, the second metal layer having a second refractive index,         wherein a ratio of the real part (n) of the second refractive         index to the imaginary part (k) of the second refractive index         is greater than or equal to 0.01 and less than or equal to 0.5.

Example 149: The pigment of Example 148, further comprising an encapsulation layer encapsulating the optical structure.

Example 150: The pigment of Example 149, wherein the encapsulation layer comprises silica.

Example 151: The pigment of any of Examples 148-150, further comprising a resin configured to chemically attach to the encapsulation layer.

Example 152: The pigment of any of Examples 148-151, configured to produce a first color at a first viewing angle and a second color different from the first color at a second viewing angle.

Example 153: The pigment of any of Examples 148-152, wherein the optical structure has a thickness, and wherein a length or a width of the optical structure is at least 5 times the thickness.

Example 154: A document comprising the pigment of any of Examples 148-153, the document comprising a main body and the pigment disposed on the main body.

Example 155: A packaging comprising the pigment of any of Examples 148-153, the packaging comprising a main body and the pigment disposed on the main body.

Example 156: The optical structure of any of Examples 1-26, configured as a foil.

Example 157: The optical structure of any of Examples 1-26, configured as a thread.

Example 158: The optical structure of any of Examples 1-26, configured as a laminate.

Example 159: The optical structure of any of Examples 110-132, included in a thread.

Example 160: The optical structure of any of Examples 110-132, included in a foil.

Example 161: The optical structure of any of Examples 110-132, included in a laminate.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments will now be described in conjunction with the drawings.

FIG. 1 schematically illustrates a side view of an optical structure configured to be used as a security feature.

FIGS. 2A-1 and 2A-2 schematically illustrate side views of optical structures configured to be used as a security feature in the form of a platelet encapsulated with an encapsulating layer, comprising, for example, a SiO₂ layer and silica spheres.

FIGS. 2B-1 and 2B-2 illustrates a plurality of platelets dispersed in a polymer which can comprise an ink or a paint medium.

FIG. 3 illustrates the silane coupling agent bonded to an exposed surface of the encapsulation layer of a platelet. Another side of the silane coupling agent can also bond to a medium such as a polymer in which the platelets are dispersed.

FIG. 4 is a schematic illustration showing propagation light incident on the optical structure and the resultant nodes in field strength at the metal layers.

FIGS. 5A and 5B illustrate transmission and reflection spectra of examples of optical structures.

FIGS. 6A-6D and 7A-7D are a* b* plots showing the color travel or change in reflection and transmission respectively for four different example optical structures.

FIGS. 8A and 8B respectively illustrate the transmittance and reflectance spectra for an example of the optical structure.

FIGS. 8C and 8D respectively illustrate the transmittance and reflectance spectrum for an example of the optical structure.

FIGS. 8E and 8F respectively illustrate the transmittance and reflectance spectrum for an example of the optical structure.

FIG. 8G illustrates the a*b* values in the CIELa*b* color space for an example of the optical structure for different viewing angles between 0 degrees and 40 degrees with respect to the normal to the surface of the example of the optical structure in transmission mode.

FIG. 8H illustrates the a*b* values in the CIELa*b* color space for an example of the optical structure for different viewing angles between 0 degrees and 40 degrees with respect to the normal to the surface of the example of the optical structure in reflection mode.

FIG. 9A schematically illustrates a cross-sectional view of an embodiment of an optical structure configured to be used as a security feature. FIG. 9B schematically illustrates a cross-sectional view of another embodiment of an optical structure configured to be used as a security feature.

FIG. 10 is a schematic illustration of a laminate structure comprising an optical structure that is affixed to a banknote.

FIG. 11A shows a banknote with two windows, each window including a different optical structure. FIG. 11B shows a security device with two at least partially overlapping windows, each window comprising a different optical structure.

FIGS. 12 and 13 illustrate examples of a security device comprising an optical structure disposed under or over a text, symbol or number. The text, symbol or number becomes visible when the viewing angle is changed.

DETAILED DESCRIPTION

To curtail counterfeiting, currency, documents (e.g., banknotes) as well as other items such as products and packaging can be provided with security features that can be inspected by the general public to verify authenticity. In many cases, it can be advantageous if the security features can be easily seen under a variety of light conditions and without the need for special lighting conditions. It can also be desirable that the security features have distinct characteristics that can be easily identified by the public within a 1-10 second time frame. In addition, it is advantageous in general, if the security feature is not susceptible to copying by electronic or photographic equipment, such as, for example, printers, copiers, cameras, etc.

One example of a security feature employed in banknotes is the watermark, which has a fairly high degree of awareness among the general public. An example of a watermark can be an image comprising light and dark regions that can be easily seen by holding up the banknote to see the watermark in light transmission. However, watermarks may be susceptible to be copied and thus are not very secure. Other examples of security features may use inks and motion type features that are not readily seen under low light conditions (e.g., at low lit bars, restaurants, etc.), have poor image resolution, and/or have slow optical movement relative to the movement of the banknote. Accordingly, some existing security features tend to be more complicated structures having more complex color changing effects. This approach, however, can be disadvantageous when the complicated security devices are applied to banknotes or currency, as these complicated security devices may confuse an average person who is looking for a distinctive security feature.

Having a security features that has high contrast with respect to the background that can be easily identified by the general public under a variety of light conditions, including low light, can be advantageous. Accordingly, various security features disclosed can appears to have one color in reflection and another different color in transmission. These security features can be incorporated in a banknote. A consumer, merchant, or a bank teller can holdup such a banknote to light to readily verify the authenticity of the banknote. Additionally, in some implementations, the security feature can be configured to exhibit color shift and/or movement of identifiable features when the viewing angle is varied to enhance security. These and other features are described in further detail herein.

Accordingly, various security features contemplated herein can comprise optical stacks and/or structures that are at least partially reflective and at least partially transmissive. The security features contemplated herein can be configured as coatings, threads, laminates, foils, films, pigments and/or inks and incorporated with banknotes or other items. The innovative aspects described in this application also include systems and methods of fabricating optical structures and/or stacks that are at least partially reflective and at least partially transmissive. In some embodiments, such optical structures may be fabricated on support or base layers or sheets such as webs (e.g., roll coated webs). Processes described herein may also include removing the fabricated optical structures and/or stacks from a support or base layer (e.g., roll or sheet). The innovative aspects described in this application further includes methods and systems for including the optical structures and/or stacks that are at least partially reflective and at least partially transmissive in pigment and inks having a desired amount of durability and mechanical strength to be further used in or on or incorporated into banknotes and other security devices/documents.

FIG. 1 schematically illustrates an optical structure 10 comprising a stack of layers that can be used as a security feature. The optical structure 10 comprises at least two metal layers 13 and 15. The at least two metal layers 13 and 15 can comprise metals having a ratio of the real part (n) of the refractive index to the imaginary part (k) of the refractive index (k) that is less than 1. For example, the at least two metal layers 13 and 15 can comprise metals that have an n/k value between about 0.01 and about 0.6, between about 0.015 and about 0.6, between about 0.01 and about 0.5, between about 0.01 and about 0.2, between about 0.01 and about 0.1, or any value in a range or sub-range defined by these values. Accordingly, the at least two metal layers 13 and 15 can comprise silver, silver alloys, gold, aluminum or copper and their respective alloys. Nickel (Ni) and Palladium (Pd) can be used in some implementations. In some cases, however, the at least two metal layers 13 and 15 do not comprise chromium, titanium, and/or tungsten or any metal having an n/k ratio greater than 0.6. In some cases, the metal layer 13 and 15 can have a thickness greater than or equal to about 3 nm and less than or equal to about 35 nm. For example, thickness of the metal layer 13 and 15 can be greater than or equal to about 10 nm and less than or equal to about 30 nm, greater than or equal to about 15 nm and less than or equal to about 27 nm, greater than or equal to about 20 nm and less than or equal to about 25 nm, or any value in a range or sub-range defined by these values. The thickness of the metal layer 13 can be equal to the thickness of the metal layer 15. Alternately, the thickness of the metal layer 13 can be greater than or less than the thickness of the metal layer 15.

A transparent dielectric layer 14 is sandwiched between the at least two metal layers 13 and 15. The dielectric layer 14 can have a refractive index greater than, less than or equal to 1.65. Materials with an index greater than or equal to 1.65 can be considered as high refractive index materials for the purpose of this application and materials with an index less than 1.65 can be considered as low index materials for the purpose of this application. The transparent dielectric layer 14 can comprise inorganic materials including but not limited to silicone dioxide (SiO₂), aluminum oxide (Al₂O₃), magnesium fluoride (MgF₂), cerium fluoride (CeF₃), lanthanum fluoride (LaF₃), zinc oxide (ZnO), zinc sulfide (ZnS), zirconium dioxide (ZrO₂), titanium dioxide (TiO₂), tantalum pentoxide (Ta₂O₅), ceric oxide (CeO₂), ytterium oxide (Y₂O₃), indium oxide (In2O₃), tin oxide (SnO2), indium tin oxide (ITO) and tungsten trioxide (WO₃) or combinations thereof. The transparent dielectric layer 14 can comprise polymers including but not limited to parylene, acrylates, and/or methacrylate. Without any loss of generality, the transparent dielectric layer 14 can comprise a material having an index of refraction greater than, less than, or equal to 1.65 and an extinction coefficient between 0 and about 0.5 such that it has low absorption of light in the visible spectral range.

The dielectric layer 14 can have a thickness that is greater than or equal to about 75 nm and less than or equal to about 2 micron. For example, the dielectric layer 14 can have a thickness that is greater than or equal to about 150 nm and less than or equal to about 650 nm, greater than or equal to about 200 nm and less than or equal to about 600 nm, greater than or equal to about 250 nm and less than or equal to about 550 nm, greater than or equal to about 300 nm and less than or equal to about 500 nm, greater than or equal to about 350 nm and less than or equal to about 450 nm, greater than or equal to about 700 nm and less than or equal to about 1 micron, greater than or equal to about 900 nm and less than or equal to about 1.1 micron, greater than or equal to about 1 micron and less than or equal to about 1.2 micron, greater than or equal to about 1.2 micron and less than or equal to about 2.0 microns or any value in a range/sub-range defined by these values. Without subscribing to any particular theory, in various implementations, the thickness of the dielectric layer 14 can be approximately a quarter wavelength of light (e.g., visible light) incident thereon or an integer multiple of a quarter wavelength. In various implementations, the thickness of the dielectric layer 14 may be, for example, 1/4, 3/4, 5/4, 7/4, 9/4, 10/4, etc. of the wavelength of visible light incident on the dielectric layer 14.

The optical structure 10 further comprises a transparent dielectric layer 12 that is disposed on a side of the metal layer 13 that is opposite to the dielectric layer 14 and a transparent dielectric layer 16 that is disposed on a side of the metal layer 15 that is opposite to the dielectric layer 14. In some cases, layers 12 and 16 can comprise materials having a refractive index greater than or equal to 1.65. For example, layers 12 and 16 can comprise ZrO₂, TiO₂, ZnS, ITO (indium tin oxide). CeO₂ or Ta₂O₃. Dielectric layers 12 and 16 can have a thickness that is greater than or equal to about 100 nm and less than or equal to about 400 nm, greater than or equal to about 150 nm and less than or equal to about 350 nm, greater than or equal to about 200 nm and less than or equal to about 300 nm, or any value in a range/sub-range defined by these values. The thickness of the dielectric layer 12 can be equal to the thickness of the dielectric layer 16. Alternately, the thickness of the dielectric layer 12 can be greater than or less than the thickness of the dielectric layer 16. The optical structure 10 can have a thickness that is less than or equal to about 2 microns.

Fabricating the optical structure 10 can include providing the layer of dielectric material 12 (or the layer of dielectric material 16) and depositing the metal layer 13 (or the metal layer 15) over the layer of dielectric material 12 (or the layer of dielectric material 16). The metal layer 13 (or the metal layer 15) can be deposited over the layer of dielectric material 12 (or the layer of dielectric material 16) using an electroless method discussed in further detail below. The metal layer 13 (or the metal layer 15) can be deposited as a continuous thin film, as small spheres, metallic clusters or island like structures. The other dielectric layer 14 can be subsequently disposed over the metal layer 13 (or the metal layer 15). The initial layer of dielectric material 12 (or the layer of dielectric material 16) can be disposed and/or formed over a support. The support is also referred to herein as a base layer. The support can comprise a carrier. The support can comprise a sheet such as a web. The support can comprise a substrate. The substrate can be a continuous sheet of PET or other polymeric web structure. The support can comprise a non-woven fabric. Non-woven fabrics can be flat, porous sheets comprising fibers. In some implementations, the non-woven fabric can be configured as a sheet or a web structure that is bonded together by entangling fiber or filaments mechanically, thermally, or chemically. In some implementations, the non-woven fabric can comprise perforated films (e.g., plastic or molten plastic films). In some implementations, the non-woven fabric can comprise synthetic fibers such as polypropylene or polyester or fiber glass.

The support can be coated with a release layer comprising a release agent. The release agent can be soluble in solvent or water. The release layer can be polyvinyl alcohol, which is water soluble or an acrylate which is soluble in a solvent. The release layer can comprise a coating, such as, for example, salt (NaCl) or cryolite (Na₃AlF₆) deposited by evaporation before the layers of the optical structure are deposited/formed.

In some implementations of the support configured as a non-woven fabric, the non-woven fabric can be coated with a release layer. Such implementations can be dipped or immersed in a solvent or water that acts as a release agent to dissolve or remove the release layer. The release agent (e.g., the solvent or water) is configured to penetrate from a side of the non-woven fabric opposite the side on which the optical structure is disposed to facilitate release of the optical structure instead of having to penetrate through the optical structure. The optical structure is recovered from the solvent or water after dissolution of the release layer. In some manufacturing approaches, the recovered optical structure can then be processed into a pigment.

In one method of fabrication, the optical structure 10 can be fabricated, for example, deposited or formed on a coated web, a coated base layer, a coated carrier or a coated substrate. The coating on the web, the base layer, the substrate or the carrier can be configured as a release layer to facilitate easy removal of the optical structure 10.

The optical structure 10 can be configured as a film or a foil by disposing over a substrate or other support layer having a thickness, for example, greater than or equal to about 10 microns and less than or equal to about 25 microns. For example, a substrate or support layer such as a polyester substrate or support layer can have a thickness greater than or equal to 12 microns and less than or equal to 22.5 microns, greater than or equal to 15 microns and less than or equal to about 20 microns. The substrate or support layer can comprise materials, such as, for example, polyester, polyethylene, polypropylene, or polycarbonate. The support or support layer itself can be dissolvable. The support or support layer, for example, can also comprise polyvinyl alcohol, which can be dissolved, for example, in water. Accordingly, instead of using a release layer on a insoluble support web, the support web itself may comprise soluble material. Accordingly, the support or support layer can be dissolved leaving the optical coating remaining. The optical structure 10 configured as a film or a foil can be encapsulated with a polymer, such as, for example a UV cured polymer.

The optical structure 10 can comprises additional layers. For example, a thin protective layer may be disposed between the metal layer 13 and the dielectric layer 12 and/or between the metal layer 15 and the dielectric layer 16. The protective layer can comprise materials, such as, for example, NiCrO_(x), Si₃N₄, CeSnO₄ and ZnSnO₄. The protective layers can have a thickness between about 3-5 nm. The protective layers can advantageously increase the durability of the metal layers 13 and 15.

Instead of a film, the optical structure, 10, may be removed from the substrate, web, carrier, or support layer on which it is fabricated and divided into platelets having a size that is suitable for a pigment or printing ink. Platelets having a size that is suitable for a pigment or printing ink can have an area, length, and/or width that is about 5-10 times the thickness of the platelet, in some implementations. Accordingly, the platelets having a thickness of about 1 micron, and/or can have a width and/or a length that is between approximately 5 micron and about 50 microns. For example, the width and/or a length can be greater than or equal to about 5 micron and less than or equal to about 15 microns, greater than or equal to about 5 microns and less than or equal to about 10 microns, greater than or equal to about 5 micron and less than or equal to about 40 microns, greater than or equal to about 5 microns and less than or equal to about 20 microns, or any value in the ranges/sub-ranges defined by these values. Platelets having a length and/or width that is less than about 5-10 times the thickness of the platelet, such as, for example having a length and/or width that is equal to the thickness of the platelet can be oriented along their edges in the printing ink or pigment. This can be disadvantageous since pigment or printing ink comprising platelets that are oriented along their edges may not exhibit the desired colors in reflection and transmission modes. Dimensions such as, thicknesses, lengths and/or widths outside these ranges are also possible.

FIG. 2A-1 illustrates an example of a platelet 20. The optical structure, 10 is fractured, cut, diced or otherwise separated to obtain the separate, for example, microns sized, pieces or platelets. In some implementations, the obtained platelets may be surrounded by an encapsulating layer 21. The encapsulating layer 21 can comprise a moisture resistant material, such as, for example silicon dioxide. The encapsulating layer 21 can also comprise silica spheres 22 and 23. The silica spheres 22 and 23 can be of the same size or have different sizes. The encapsulating layer 21 can help protect the at least two metal layers 13 and 15 from corrosion. The encapsulating layer 21 can additionally and/or alternatively reduce the occurrence of delamination of the at least two metal layers 13 and 15 from the other layers of the optical structure 10. The optical structures 10 surrounded by the encapsulating layer 21, and potentially comprising the silica spheres 22 and 23, can be configured as platelets 20 that are suitable for a pigment or printing ink. The silica spheres 22 and 23 of the encapsulating layer 21 can help prevent the platelets from adhering to one another. Without the spheres the platelets may stick together like two microscope slides stick together. The spheres 22 and 23 can also prevent the platelets 20 from sticking to the print rollers in the printing machine. One method of surrounding the optical structure 10 with an encapsulating layer 21 can rely on sol-gel technology using tetraethylorthosilicate (TEOS). In one method of forming the encapsulating layer 21, an alcohol based solution of TEOS can be added in small quantities (e.g., one or more drops at a time) to a dispersion of the platelets in alcohol or water. A catalyst, such as, for example, an acid or sodium hydroxide solution can be added into the a dispersion of the platelets in alcohol or water in small quantities (e.g., one or more drops at a time). The dispersion of the platelets in alcohol or water can be heated to a temperature of about 50-70° C., while stirring to transform TEOS to a silica coating. Other processes, however, may be employed.

In some embodiments, a plurality of platelets 20 can form a pigment. Such a pigment may be color shifting (e.g., the color reflected and/or transmitted changes with angle of view or angle of incidence of light), in some cases. In some embodiments, non-color shifting pigment or dye may be mixed with the pigment. In some embodiments other materials may be included with the platelets 20 to form the pigment. Although some of the pigments discussed herein can provide color shift with change in viewing angle or angle of incidence of light, pigments that do not exhibit color shift with change in viewing angle or angle of incidence of light or that produce very little color shift with change in viewing angle or angle of incidence of light are also contemplated.

In some embodiments, the platelets 20 can be added to a medium such as a polymer 25 (e.g., a polymeric resin) to form a dichroic ink, a pigment, or paint as shown in FIG. 2B-1 . The platelets can be suspended in the medium (e.g., polymer) 25. The platelets can be randomly oriented in the medium (e.g., polymer) 25 as shown in FIG. 2B-1 . During the printing process, in some cases, the individual platelets can be oriented parallel to the surface of the object (e.g., paper) to which the pigment, the paint, or the dichroic ink is being applied as a result of, for example, the printing action, gravity, and/or surface tension of the normal drying process of the pigment, the paint, or the dichroic ink as shown in FIG. 2B-2 . The medium 25 can comprise material including but not limited to acrylic melamine, urethanes, polyesters, vinyl resins, acrylates, methacrylate. ABS resins, epoxies, styrenes and formulations based on alkyd resins and mixtures thereof. In some implementations, the medium 25. e.g., polymer, can have a refractive index that closely matches the refractive index of the encapsulating silica layer 21 and/or silica balls such that the encapsulating layer and/or the silica balls do not adversely affect the optical performance of the pigment, the paint, or the dichroic ink in the medium.

In various implementations, the platelets 20 need not be surrounded by an encapsulating layer. In such implementations, one or more platelets 20 that are not encapsulated by an encapsulating layer can be added or mixed with an ink or a pigment medium (e.g., varnish, polymeric resin, etc.) to obtain a dichroic ink or pigment as discussed above. In various implementations, the dichroic ink or pigment can comprise a plurality of platelets 20. The optical structures 10 that are configured as the plurality of platelets 20 can have different distributions of shapes, sizes, thicknesses and/or aspect ratios. The optical structures 10 that are configured as the plurality of platelets 20 can also have different optical properties. For example, the optical structures 10 that are configured as the plurality of platelets 20 can also have different color properties.

In some implementations, an optical structure comprising only the metal layers 13 and 15 and the transparent dielectric layer 14 without the high refractive index dielectric layers 12 and 16 as depicted in FIG. 2A-2 can be configured as platelets as discussed above and dispersed in the medium 25 as shown in FIG. 2B-2 to manufacture a dichroic printing ink, paint or pigment as discussed above. In some implementations, the platelets including an optical structure comprising only the metal layers 13 and 15 and the transparent dielectric layer 14 without the high refractive index dielectric layers 12 and 16 need not be encapsulated in an encapsulating layer as discussed above.

A silane coupling agent can be bonded to the encapsulating layer 21 to form a functionalized platelet 30 as shown in FIG. 3 . Bonding of the silane coupling agent to the encapsulating layer can occur through a hydrolyzing reaction. The silane coupling agent can bind to the polymer (e.g., polymeric resin) of the printing ink or paint medium so that the heterogeneous mixture of pigment and the polymer do not separate during the printing process and substantially function in much the same way as a homogeneous medium would function. The printing ink or paint medium can comprise material including but not limited to acrylic melamine, urethanes, polyesters, vinyl resins, acrylates, methacrylate, ABS resins, epoxies, styrenes and formulations based on alkyd resins and mixtures thereof. The silane coupling agents used can be similar to the silane coupling agents sold by Gelest Company (Morristown, Pa. USA). In some implementations, the silane coupling agent can comprise a hydrolyzable group, such as, for example, an alkoxy, an acyloxy, a halogen or an amine. Following a hydrolyzing reaction (e.g., hydrolysis), a reactive silanol group is formed, which can condense with other silanol groups, for example, with the silica spheres of the encapsulating layer 21 or the encapsulating layer of silica to form siloxane linkages. The other end of the silane coupling agent comprises the R-group 31. The R-group 31 can comprise various reactive compounds including but not limited to compounds with double bonds, isocyanate or amino acid moieties. Reaction of the double bond via free radical chemistry can form bonds with the ink polymer(s) such as those based on acrylates, methacrylates or polyesters based resins. For example, isocyanate functional silanes, alkanolamine functional silanes and aminosilanes can form urethane linkages.

Without any loss of generality, in various implementations of the optical structure 10 configured as a platelet that do not comprise the encapsulating layer, the silane coupling agent can be bonded to one or both of the high refractive index dielectric layers 12 and 16 comprising a dielectric material (e.g., TiO₂) suitable to be bonded with the silane coupling agent.

Without any loss of generality, the optical structure 10 can be considered as an interference stack or cavity. Ambient light incident on the surface of the optical structure 10 is partially reflected from the various layers of the optical structure 10 as shown by rays 47 and 48 in FIG. 4 and partially transmitted through the various layers of the optical structure 10 as shown by ray 49 in FIG. 4 . FIG. 4 illustrates an embodiment of an optical structure 10 comprising the high refractive index dielectric layer 12 and 16, metal layers 13 and 15 and a dielectric layer 14 encapsulated in the encapsulating layer 21. Some wavelengths of the ambient light reflected from the various layers may interfere constructively and some other wavelengths of the ambient light reflected from the various layers may interfere destructively. Similarly, some wavelengths of light transmitted through the various layers may interfere constructively and some other wavelengths of the ambient light transmitted through the various layers may interfere destructively. As a result of which, the optical structure 10 appears colored when viewed in transmission and reflection mode. In general, the color and the intensity of light reflected by and transmitted through the optical structure 10 can depend on the thickness and the material of the various layers of the optical structure 10. By changing the material and the thickness of the various layers, the color and intensity of light reflected by and transmitted through the optical structure 10 can be varied. Without subscribing to any particular scientific theory about the operation of the optical structures 10, in general, the material and the thickness of the various layers can be configured such that some or all of the ambient light reflected by the various layers interfere such that a node 45 in the field 42 occurs at the two metal layer 13 and 15. Without subscribing to any particular scientific theory, it is noted that in some cases those wavelengths that are substantially equal to the thickness of the spacer layer (e.g., wavelengths within about ±10% of the thickness of the spacer layer) will interfere such that a node 45 in the field 42 occurs at the two metal layer 13 and 15. For other wavelengths, a node 45 might not occur. Accordingly, in some implementations, the two metal layers 13 and 15 might not be visible in the reflection mode. Again, without subscribing to a particular scientific theory, based on the thickness of the two metal layers 13 and 15 and the transparent dielectric layer 14, a portion of the incident light may be transmitted through the optical structure 10 as a result of the phenomenon of “induced transmittance” or “induced transmission”. The reflection and transmission spectral characteristics are discussed below.

FIG. 5A shows a spectral plot in both transmission (curve 501 a) and reflection (curve 503 a) for a first example of the optical structure 10. The materials of the various layers of the first example of the optical structure 10 and the thickness of the various layers of the first example of the optical structure 10 are provided in Table 1 below. As indicated in Table 1, the first example of the optical structure 10 comprises two metal layers comprising silver. The two silver layers correspond to the at least two metal layer 13 and 15 of the optical structure 10 shown in FIG. 1 . Both the silver layers have the same thickness of 25 nm. A dielectric layer having a thickness of 300 nm is sandwiched between the two silver layers. The dielectric layer comprises SiO₂ which has a refractive index of 1.47011. The dielectric layer comprising SiO₂ corresponds to the transparent layer 14 having a low refractive index (i.e., refractive index less than 1.65). A layer of ZrO₂ is disposed on the side of each of the two silver layers that is opposite the side facing the SiO₂ layer. Each of the two layers comprising ZrO₂ has a thickness of 150 nm. As noted from Table 1 below, ZrO₂ has a refractive index of 2.27413. The two layers comprising ZrO₂ corresponds to the transparent layers 12 and 16 having a high refractive index (i.e., refractive index greater than or equal to 1.65). The first example of the optical structure 10 is encapsulated in a SiO₂ matrix as indicated in Table 1. The SiO₂ matrix is used to simulate the printing medium or ink which has a similar refractive index.

The transmission and reflection of light observed at an angle of 0 degrees with respect to a normal to the first example of the optical structure 10 is shown in FIG. 5A. The reflection spectrum 503 a (indicated as curve #1 in FIG. 5A) and the transmission spectrum 501 a (indicated as curve #0 in FIG. 5A) in the spectral range between about 400 nm and about 700 nm which includes the visible spectral range were obtained using a simulation software from http://thinfilm.hansteen.net.

Parameters

TABLE 1 Parameters of a first example of the optical structure that has the reflection and transmission spectra as shown in FIG. 5A. Curve #0 # # Slab: # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # ZRO2 d = 1.5e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # AG d-2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK # SIO2 d = 3e−07 N = (1.47011 , 0) mynkdb/SIO2.NK # AG d = 2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK # ZRO2 d = 1.5e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # # Beam: # Wavelength = (4e−07, 0) Angle = 0.0174533 Polarization = 1 N = (1.47011, 0) # # Supported spectral range: 2.5e−07m-8.5e−07m. # # Lambda[nm] R[ ] # Curve #1 # # Slab: # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # ZRO2 d = 1.5e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # AG d = 2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK # SIO2 d = 3e−07 N = (1.47011 , 0) mynkdb/SIO2.NK # AG d = 2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK # ZRO2 d = 1.5e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # # Beam: # Wavelength = (4e−07, 0) Angle = 0.0174533 Polarization = 1 N = (1.47011, 0) # # Supported spectral range: 2.5e−07m-8.5e−07m. # # Lambda[nm] T[ ] #

It can be seen from FIG. 5A that the transmission curve 501 a (curve #0) has a peak with a maximum value occurring at a wavelength of about 520 nm and the reflection curve 503 a has two peaks with a first maximum value occurring at a wavelength of 420 nm and a second maximum value occurring at a wavelength of about 650 nm. The maximum value of the transmission and reflection peaks is greater than 0.5 which indicates that the transmission and reflection peaks have high intensities. Furthermore, the transmission and reflection peaks have a bandwidth as measured at 50% of the maximum value of the peak greater than about 20 nm. The bandwidth as measured at 50% of the maximum value of the peak is referred to as full width at half maximum (FWHM). It is observed from FIG. 5A that the FWHM of the transmission peak is about 75 nm.

Based on the position of the transmission and reflection peaks and the bandwidth of the transmission and reflection peaks, the optical structure 10 can be perceived as having a first color in the reflection mode and a second color in the transmission mode by an average human eye. In some cases, the first color and the second color can be complimentary colors. In some cases, the transmission and reflection peaks comprising a range of wavelengths of the visible spectral range can have a high intensity and a FWHM greater than 2 nm (e.g., FWHM greater than or equal to about 10 nm. FWHM greater than or equal to about 20 nm, FWHM greater than or equal to about 30 nm. FWHM greater than or equal to about 40 nm, FWHM greater than or equal to about 50 nm, FWHM greater than or equal to about 60 nm, FWHM greater than or equal to about 70 nm. FWHM greater than or equal to about 100 nm, FWHM greater than or equal to about 200 nm, FWHM less than or equal to about 300 nm. FWHM less than or equal to about 250 nm, or any value in a range/sub-range defined by these values).

The one or more reflection peaks can be considered to have a high intensity if the reflectivity or reflectance of the peak in a range of visible wavelengths is greater than or equal to about 50% and less than or equal to about 100%. For example, the one or more reflection peaks can be considered to have a high intensity if the amount of light reflected or reflectivity or reflectance in a range of visible wavelengths is greater than or equal to about 55% and less than or equal to about 99%, greater than or equal to about 60% and less than or equal to about 95%, greater than or equal to about 70% and less than or equal to about 90%, greater than or equal to about 75% and less than or equal to about 85%, or any value in a range/sub-range defined by these values.

The one or more transmission peaks can be considered to have a high intensity if the transmissivity or transmittance of the peak in a range of visible wavelengths is greater than or equal to about 50% and less than or equal to about 100%. For example, the one or more transmission peaks can be considered to have a high intensity if the amount of light transmitted or transmissivity or transmittance in a range of visible wavelengths is greater than or equal to about 55% and less than or equal to about 99%, greater than or equal to about 60% and less than or equal to about 95%, greater than or equal to about 70% and less than or equal to about 90%, greater than or equal to about 75% and less than or equal to about 85%, or any value in a range/sub-range defined by these values.

The first example of the optical structure 10 having a design as depicted in Table 1 and having a reflection spectrum and a transmission spectrum as shown in FIG. 5A appears green in transmission mode and as magenta in reflection mode to an average human eye. Without any loss of generality, it can be advantageous, in various implementations, for the peaks in the reflection and transmission spectra to be non-overlapping as shown in FIGS. 5A and 5B such that a reflection peak having a highest possible reflectance or reflectivity can be obtained in one region of the visible spectral range and a transmission peak having a highest possible transmittance or transmissivity can be obtained in a non-overlapping region of the visible spectral range. Accordingly, the reflected color and the transmitted color can be different and potentially complementary to each other, such as, for example, red and green, yellow and violet, blue and orange, green and magenta, etc.

The shape of the transmission and reflection peaks, the position of the maximum of the transmission and reflection peaks, the FWHM of the transmission and reflection peaks, etc. can be varied by varying the materials and/or thickness of the various layers of the optical structure 10. This can be observed from FIG. 5B which depicts the reflection spectrum 503 b and transmission spectrum 501 b of a second example of the optical structure 10 which has the same material composition as the first example of the optical structure 10 but different thickness for the various layers. The parameters of the second example of the optical structure 10 are provided in Table 2 below. As noted from Table 2, the thickness of the dielectric layer comprising SiO₂ and having a refractive index of 1.47011 in the second example of the optical structure 10 is 400 nm instead of 300 nm in the first example of the optical structure 10. Furthermore, the thickness of the two ZrO₂ disposed on either side of each of the two silver layers is 225 nm in the second example of the optical structure 10 instead of 150 nm in the first example of the optical structure 10.

Parameters

TABLE 2 Parameters of a second example of the optical structure that has the reflection and transmission spectra as shown in FIG. 5B. Curve #0 # # Slab: # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # ZRO2 d = 2.25e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # AG d = 2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK # SIO2 d = 4e−07 N = (1.47011 , 0) mynkdb/SIO2.NK # AG d = 2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK # ZRO2 d = 2.25e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # # Beam: # Wavelength = (4e−07, 0) Angle = 0.0174533 Polarization = 1 N = (1.47011, 0) # # Supported spectral range: 2.5e−07m-8.5e−07m. # # Lambda[nm] R[ ] # Curve #1 # # Slab: # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # ZRO2 d = 2.25e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # AG d = 2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK # SIO2 d = 4e−07 N = (1.47011 , 0) mynkdb/SIO2.NK # AG d = 2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK # ZRO2 d = 2.25e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # # Beam: # Wavelength = (4e−07, 0) Angle = 0.0174533 Polarization = 1 N = (1.47011, 0) # # Supported spectral range: 2.5e−07m-8.5e−07m. # # Lambda[nm] T[ ] #

As a result of the change in the thickness of the dielectric layers comprising SiO₂ and ZrO₂ between the second example of the optical structure and the first example of the optical structure, an average eye would perceive the second example of the optical structure to appear green in reflection mode and a magenta in transmission mode when viewed along a direction normal to the surface of the second example of the optical structure.

The color of the first example and the second example of the optical structure 10 as perceived by the average human eye in reflection mode and transmission mode can shift from the above described magenta and green colors at different viewing angles with respect to the normal to the surface of the first example and the second example of the optical structure 10. For example, the first example of the optical structure 10 can appear yellowish green in reflection mode and blue in transmission mode when viewed at an angle of about 35 degrees with respect to the normal to the surface of the first example of the optical structure 10. As another example, the second example of the optical structure 10 can appear pale purple in reflection mode and yellowish in transmission mode when viewed at an angle of about 35 degrees with respect to the normal to the surface of the second example of the optical structure 10. Without any loss of generality, the reflection and the transmission peaks can exhibit a blue shift towards shorter wavelengths as the viewing angle with respect to the normal to the surface of the first example and the second example of the optical structure 10 increases.

TABLE 3 CIELab values for transmission mode when the first example of the optical structure having parameters as described in Table 1 is viewed at different viewing angles in the presence of a D65 light source. Incident Angle L* a* b* 0.0 66.0433 −91.9989 11.4335 Design: First Example 5.0 65.5578 −91.5328 9.3070 of the Optical Structure 10.0 64.0035 −89.0283 2.6936 Polarization: P 15.0 61.1497 −81.1844 −8.9303 Source: D65 20.0 56.8304 −63.3282 −25.7758 Observer: CIE 1931 25.0 51.2146 −32.8229 −46.6651 Mode: Transmittance 30.0 44.8902 5.7777 −67.7337 35.0 38.6590 39.5335 −81.9630 40.0 33.4474 53.5162 −81.6652 45.0 30.4059 43.0007 −64.1869

TABLE 4 CIELab values for reflection mode when the first example of the optical structure having parameters as described in Table 1 is viewed at different viewing angles in the presence of a D65 light source. Incident Angle L* a* b* 0.0 79.2753 51.6407 −11.0765 Design: First Example of 5.0 79.6541 50.6966 −9.6957 the Optical Structure 10.0 80.8290 47.4222 −5.3025 Polarisation: P 15.0 82.8379 40.8204 2.7687 Source: D65 20.0 85.5358 30.2258 15.3945 Observer: CIE 1931 25.0 88.5026 16.2157 33.3659 Mode: Reflectance 30.0 91.2316 1.0176 55.5312 35.0 93.4068 −11.0169 70.1468 40.0 94.9289 −14.7597 57.7563 45.0 95.7892 −10.6419 32.4479

Tables 3 and 4 above provide the CIELa*b* values for transmission mode and reflection mode respectively when the first example of the optical structure having parameters as described in Table 1 is viewed at different viewing angles in the presence of a D65 light source. Tables 5 and 6 below provide the CIELa*b* values for transmission mode and reflection mode respectively when the second example of the optical structure having parameters as described in Table 2 is viewed at different viewing angles in the presence of a D65 light source. The CIELab color closely represent the colors perceived by an average human eye. The CIELab color space mathematically describe various colors perceived by an average human eye in the three dimensions L for lightness, a for the color component green-red, and b for the color component from blue-yellow. The a-axis extends longitudinally in a plane from green (represented by −a) to red (represented by +a). The b-axis extends along a transverse direction in the plane perpendicular to the a-axis from blue (represented by −b) to yellow (represented by +b). The brightness is represented by the L-axis which is perpendicular to the a-b plane. The brightness increases from black represented by L=0 to white represented by L=100. The CIELab values for different viewing angles using a D65 illuminant were calculated using Essential Macleod Thin Film Software.

TABLE 5 CIELab values for transmission mode when the second example of the optical structure having parameters as described in Table 2 is viewed at different viewing angles in the presence of a D65 light source. Incident Angle L* a* b* 0.0 35.3624 87.7761 −73.0966 Design: Second Example of the 5.0 35.9375 88.1214 −71.4170 Optical Structure 10.0 37.8504 88.3232 −65.5105 Polarization: P 15.0 41.5481 86.2320 −53.1339 Source: D65 20.0 47.3489 79.0290 −32.0276 Observer: CIE 1931 25.0 54.8227 62.6584 −2.6495 Mode: Transmittance 30.0 62.6567 31.6730 29.2861 35.0 68.8117 −13.6155 53.1104 40.0 70.1939 −60.8762 56.3246 45.0 63.8734 −83.2865 29.4710

TABLE 6 CIELab values for reflection mode when the second example of the optical structure having parameters as described in Table 2 is viewed at different viewing angles in the presence of a D65 light source. Incident Angle L* a* b* 0.0 95.0631 −31.7647 48.4548 Design: Second Example of 5.0 94.9402 −32.7902 47.4892 the Optical Structure 10.0 94.5010 −35.8118 43.8268 Polarisation: P 15.0 93.5195 −40.5801 35.7606 Source: D65 20.0 91.6012 −45.9635 22.4005 Observer: CIE 1931 25.0 88.3120 −46.8681 5.3389 Mode: Reflectance 30.0 83.5384 −31.2961 −12.0407 35.0 78.2978 5.6475 −26.1375 40.0 76.3297 41.2278 −30.5320 45.0 81.1875 43.5513 −17.6926

The optical performance of two additional examples of optical structures having parameters provided in Tables 7 and 8 were analyzed. The additional examples of optical structures were designed using Essential Macleod Thin Film Software. The material composition and the thickness of the various layers for the third example of the optical structure are provided in Table 7 and the material composition and the thickness of the various layers for the fourth example of the optical structure are provided in Table 8.

TABLE 7 Material Composition and thickness of the various layers of the third example of the optical structure 10. Optical Physical Thickness Ex- (Full Re- tinction Wavelength Thick- fractive Coef- Optical ness Layer Material Index ficient Thickness) (nm) SiO2 1.46180 0.00000 1 ZrO2 1.00000 2.06577 0.00004 1.00000000 246.88 2 Ag 1.00000 0.05100 2.96000 0.00250000 25.00 3 SiO2 1.00000 1.46180 0.00000 0.50000000 174.44 4 Ag 1.00000 0.05100 2.96000 0.00250000 25.00 5 ZrO2 1.00000 2.06577 0.00004 1.00000000 246.88 Substrate Glass 1.52083 0.00000 Total Thickness 2.50500000 718.21

TABLE 8 Material Composition and thickness of the various layers of the fourth example of the optical structure 10. Optical Physical Thickness Ex- (Full Re- tinction Wavelength Thick- fractive Coef- Optical ness Medium Material Index ficient Thickness) (nm) SiO2 1.46180 0.00000 1 ZrO2 1.00000 2.06577 0.00004 0.50000000 123.44 2 Ag 1.00000 0.05100 2.96000 0.00250000 25.00 3 SiO2 1.00000 1.46180 0.00000 0.75000000 261.66 4 Ag 1.00000 0.05100 2.96000 0.00250000 25.00 5 ZrO2 1.00000 2.06577 0.00004 0.50000000 123.44 Substrate Glass 1.52083 0.00000 Total Thickness 1.75500000 558.55

The material composition of the various layers of the third and the fourth example of the optical structure 10 is the same as the material composition of the various layers of the first and the second example of the optical structure 10. For example, similar to the first and the second example of the optical structure 10, the third and the fourth examples of the optical structure 10 comprise a SiO₂ layer sandwiched by two silver layers with ZrO₂ layers disposed on the side of the two silver layers opposite the side facing the SiO₂ layer. However, the thickness of the various layers is different for each of the first, second, third and fourth examples of the optical structure 10.

The third example of the optical structure 10 comprises two silver layers having a thickness of 25 nm each sandwiching a dielectric layer having a thickness of 174.44 nm and comprising SiO₂. The third example of the optical structure 10 comprises a layer of ZrO₂ on the side of the silver layers opposite the side facing the SiO₂ layer. Each ZrO₂ layer has a thickness of 246.88 nm. The total thickness of the third example of the optical structure 10 is 718.21 nm.

The fourth example of the optical structure 10 comprises two silver layers having a thickness of 25 nm each sandwiching a dielectric layer having a thickness of 261.66 nm and comprising SiO₂. The fourth example of the optical structure 10 comprises a layer of ZrO₂ on the side of the silver layers opposite the side facing the SiO₂ layer. Each ZrO₂ layer has a thickness of 123.44 nm. The total thickness of the fourth example of the optical structure 10 is 558.55 nm.

FIG. 6A illustrates the a*b* values in the CIELa*b* color space for the first example of the optical structure 10 having parameters as described in Table 1 for different viewing angles between 0 degrees and 45 degrees with respect to the normal to the surface of the first example of the optical structure 10 in reflection mode. It is observed from FIG. 6A that at a viewing angle of 0 degrees with respect to the normal to the surface of the first example of the optical structure 10, the first example of the optical structure 10 appears magenta to an average human eye in reflection mode. As the viewing angle increases the color reflected by the first example of the optical structure 10 shifts along the curve 601 a in the direction of the arrow towards yellow.

FIG. 6B illustrates the a*b* values in the CIELa*b* color space for the second example of the optical structure 10 having parameters as described in Table 2 for different viewing angles between 0 degrees and 45 degrees with respect to the normal to the surface of the second example of the optical structure 10 in reflection mode. It is observed from FIG. 6B that at a viewing angle of 0 degrees with respect to the normal to the surface of the second example of the optical structure 10, the second example of the optical structure 10 appears yellowish green to an average human eye in reflection mode. As the viewing angle increases the color reflected by the second example of the optical structure 10 shifts along the curve 601 b in the direction of the arrow towards magenta.

FIG. 6C illustrates the a*b* values in the CIELa*b* color space for the third example of the optical structure 10 having parameters as described in Table 7 for different viewing angles between 0 degrees and 45 degrees with respect to the normal to the surface of the third example of the optical structure 10 in reflection mode. It is observed from FIG. 6C that at a viewing angle of 0 degrees with respect to the normal to the surface of the third example of the optical structure 10, the third example of the optical structure 10 appears green to an average human eye in reflection mode. As the viewing angle increases the color reflected by the third example of the optical structure 10 shifts along the curve 601 c in the direction of the arrow towards blue at 35°. The transmission color moves from red to orange as the viewing angle increases to 35°. It is noted that the various reflection and transmission color curves move counterclockwise in the various a* b* plots of FIGS. 6A-6D and 7A-7D.

FIG. 6D illustrates the a*b* values in the CIELa*b* color space for the fourth example of the optical structure 10 having parameters as described in Table 8 for different viewing angles between 0 degrees and 45 degrees with respect to the normal to the surface of the fourth example of the optical structure 10 in reflection mode. It is observed from FIG. 6D that at a viewing angle of 0 degrees with respect to the normal to the surface of the fourth example of the optical structure 10, the fourth example of the optical structure 10 appears yellow to an average human eye in reflection mode. As the viewing angle increases the color reflected by the fourth example of the optical structure 10 shifts along the curve 601 d in the direction of the arrow towards grey. In transmission the color seen at zero degrees is blue moving to magenta at 35°. This sample is configured as a dichroic film/pigment that has a very small color shift as the angle of view changes.

FIG. 7A illustrates the a*b* values in the CIELa*b* color space for the first example of the optical structure 10 having parameters as described in Table 1 for different viewing angles between 0 degrees and 45 degrees with respect to the normal to the surface of the first example of the optical structure 10 in transmission mode. It is observed from FIG. 7A that at a viewing angle of 0 degrees with respect to the normal to the surface of the first example of the optical structure 10, the first example of the optical structure 10 appears green to an average human eye in transmission mode. As the viewing angle increases the color transmitted by the first example of the optical structure 10 shifts along the curve 701 a in the direction of the arrow towards violet.

FIG. 7B illustrates the a*b* values in the CIELa*b* color space for the second example of the optical structure 10 having parameters as described in Table 2 for different viewing angles between 0 degrees and 45 degrees with respect to the normal to the surface of the second example of the optical structure 10 in transmission mode. It is observed from FIG. 7B that at a viewing angle of 0 degrees with respect to the normal to the surface of the second example of the optical structure 10, the second example of the optical structure 10 appears purple to an average human eye in transmission mode. As the viewing angle increases the color reflected by the second example of the optical structure 10 shifts along the curve 701 b in the direction of the arrow towards green.

FIG. 7C illustrates the a*b* values in the CIELa*b* color space for the third example of the optical structure 10 having parameters as described in Table 7 for different viewing angles between 0 degrees and 45 degrees with respect to the normal to the surface of the third example of the optical structure 10 in transmission mode. It is observed from FIG. 7C that at a viewing angle of 0 degrees with respect to the normal to the surface of the third example of the optical structure 10, the third example of the optical structure 10 appears red to an average human eye in transmission mode. As the viewing angle increases the color reflected by the third example of the optical structure 10 shifts along the curve 701 c in the direction of the arrow towards orange.

FIG. 7D illustrates the a*b* values in the CIELa*b* color space for the fourth example of the optical structure 10 having parameters as described in Table 8 for different viewing angles between 0 degrees and 45 degrees with respect to the normal to the surface of the fourth example of the optical structure 10 in transmission mode. It is observed from FIG. 7D that at a viewing angle of 0 degrees with respect to the normal to the surface of the fourth example of the optical structure 10, the fourth example of the optical structure 10 appears blue to an average human eye in transmission mode. As the viewing angle increases the color reflected by the fourth example of the optical structure 10 shifts along the curve 701 d in the direction of the arrow towards magenta.

The optical structures 10 are considered to be illuminated by D65 illumination for generating the curves of FIGS. 6A-6D and 7A-7D.

FIGS. 8A and 8B respectively illustrate the transmittance and reflectance spectra for the third example of the optical structure 10 having parameters as described in Table 7. As noted, from FIGS. 8A and 8B, the third example of the optical structure 10 has a peak transmittance at about 650 nm while the reflectance is substantially uniform in the spectral region between about 400 nm and about 600 nm and a dip around 650 nm.

FIGS. 8C and 8D respectively illustrate the transmittance and reflectance spectrum for the fourth example of the optical structure 10 having parameters as described in Table 8. As noted, from FIGS. 8C and 8D, the fourth example of the optical structure 10 has a peak transmittance between about 470 nm and about 480 nm while the reflectance is substantially uniform in the spectral region between about 520 nm and about 700 nm and a dip around 470 nm.

The optical performance of an additional fifth example of the optical structure 10 are analyzed. The fifth example of the optical structure 10 comprised a glass substrate, a first dielectric layer comprising CeO₂ over the substrate, a first metal layer comprising aluminum over the first dielectric layer, a second dielectric layer comprising CeO₂ over the first metal layer, a second metal layer comprising aluminum over the second dielectric layer, and a third dielectric layer comprising CeO₂ over the second metal layer. The thickness of various metal and dielectric layers can be configured to appear blue/violet in transmission at a viewing angle between about 0 degrees and about 40 degrees with respect to a normal to the surface of the fifth example of the optical structure 10 and yellow/green in reflection at viewing angles between 0 degrees and about 40 degrees with respect to a normal to the surface of the fifth example of the optical structure 10.

FIGS. 8E and 8F respectively illustrate the transmittance and reflectance spectrum for the fifth example of the optical structure 10 discussed above. FIG. 8G illustrates the a*b* values in the CIELa*b* color space for the fifth example of the optical structure 10 for different viewing angles between 0 degrees and 40 degrees with respect to the normal to the surface of the fourth example of the optical structure 10 in transmission mode. It is observed from FIG. 8G that at a viewing angle of 0 degrees with respect to the normal to the surface of the fifth example of the optical structure 10, the fifth example of the optical structure 10 appears blue to an average human eye in transmission mode. As the viewing angle increases the color reflected by the fifth example of the optical structure 10 shifts along the curve 751 a in the direction of the arrow towards violet.

FIG. 8H illustrates the a*b* values in the CIELa*b* color space for the fifth example of the optical structure 10 for different viewing angles between 0 degrees and 40 degrees with respect to the normal to the surface of the fifth example of the optical structure 10 in reflection mode. It is observed from FIG. 8H that at a viewing angle of 0 degrees with respect to the normal to the surface of the fifth example of the optical structure 10, the fifth example of the optical structure 10 appears yellow to an average human eye in reflection mode. As the viewing angle increases the color reflected by the fifth example of the optical structure 10 shifts along the curve 751 b in the direction of the arrow towards green.

Various implementations of an optical structure that can be used as a security feature can comprise a dielectric region comprising one or more dielectric materials surrounded by a partially optically transmissive or partially reflective metal layer (e.g., partially reflective and partially transmissive metal layer). For example, the optical structure can comprise a dielectric region having first and second major surfaces (e.g., top and bottom) and edges (or sides) therebetween. The partially reflective and partially transmissive metal layer can be disposed on the edges (or sides) in addition to being disposed on the first and second major surfaces (e.g., top and bottom). In various implementations, the dielectric region comprising the one or more dielectric materials is optical transmissive and in some configurations may be optically transparent. In certain implementations, the region comprising the one or more dielectric materials is surrounded by a partially optically transmissive and partially reflective metal layer. In various implementations, the one or more dielectric materials can comprise polymer, glass, oxides (e.g., SiO₂, TiO₂) or other dielectric materials. In various implementations, the dielectric region can comprise a dielectric substrate coated with a one or more dielectric materials (e.g., layers) having a refractive index equal to, less than or greater than the refractive index of the dielectric substrate. In various implementations, the dielectric region can comprise a first dielectric material (e.g., first dielectric layer) having a first refractive index surrounded by a second dielectric material (e.g., second dielectric layer) having a second refractive index. The second refractive index can be equal to, less than or greater than the first refractive index.

FIGS. 9A and 9B illustrate different embodiments of such optical structures. FIG. 9A schematically illustrates a cross-sectional view of an embodiment of an optical structure 70 a comprising a dielectric region 30 a surrounded by a partially reflective and partially transmissive metal layer 35 a. The optical structure 70 a shown in FIG. 9A has a rectilinear (e.g., rectangular) cross-section. FIG. 9B schematically illustrates a cross-sectional view of another embodiment of an optical structure 70 b comprising a dielectric region 30 b surrounded by a partially reflective and partially transmissive metal layer 35 b. The optical structure 70 b shown in FIG. 9B has a circular cross-section.

The dielectric region 30 a and/or 30 b can comprise one or more dielectric materials such as, for example, polymer, magnesium fluoride, silicon dioxide, aluminum oxide, titanium oxide, cerium oxide, any transparent oxide material, any transparent nitride material, any transparent sulfide material, glass, combinations of any of these materials or any other inorganic or organic material. The refractive index of the one or more dielectric materials in the dielectric region 30 a and/or 30 b can have a value between about 1.35 and about 2.5. For example, the refractive index of the one or more dielectric materials in the dielectric region 30 a and/or 30 b can have a value between about 1.38 and 1.48, between about 1.48 and about 1.58, between about 1.58 and about 1.78, between about 1.75 and about 2.0, between about 2.0 and about 2.25, between about 2.25 and about 2.5, or any value in any range/sub-range defined by these values. Values outside these ranges are also possible, in some implementations. The dielectric region 30 a and/or 30 b can comprise a dielectric substrate coated with a one or more dielectric materials having a refractive index equal to, less than or greater than the refractive index of the dielectric substrate. In various implementations, the dielectric region 30 a and/or 30 b can comprise a first dielectric material having a first refractive index surrounded by a second dielectric material having a second refractive index. The second refractive index can be equal to, less than or greater than the first refractive index.

In various implementations, the dielectric region 30 a and/or 30 b can be configured as a slab, flake, a sphere, spheroid, ellipsoid, disc, or any other 3-dimensional shape enclosing a volume. The dielectric region 30 a and/or 30 b may have a regular or irregular shape. For example, as shown in FIG. 9A, the dielectric region 30 a can be configured as a slab having two major surfaces 31 a and 31 b and one or more edge surfaces disposed between the two major surfaces 31 a and 31 b. In some implementations, a number of edges surfaces may be disposed between the two major surfaces 31 a and 31 b. The number of edge surfaces may, for example, be one, two, three, four, five, six, seven, eight, nine, ten, twelve, twenty, thirty, fifty, etc. or in any range between any of these values. Values outside these ranges are also possible. The major surfaces 31 a and 31 b can have a variety of shapes. For example, one or both of the major surfaces 31 a and 31 b can have a rectilinear or curvilinear shape in certain implementations. The shape may be regular or irregular in certain implementations. For example, one or both of the major surfaces 31 a and 31 b can have a square shape, a rectangular shape, a circular shape, an oval shape, an elliptical shape, pentagonal shape, a hexagonal shape, an octagonal shape or any polygonal shape. In various implementations, one or both of the major surface 31 a and 31 b can have jagged edges such that the lateral dimensions (e.g., length or width) of the one or both of the major surface 31 a and 31 b varies across the area of the one or both of the major surface 31 a and 31 b. Other configurations are also possible. Additionally, other shapes are also possible. One or more of the edge surfaces can have a variety of shapes (e.g., as viewed from the side), such as, for example, a square shape, a rectangular shape, an oval shape, an elliptical shape, a pentagonal shape, a hexagonal shape, an octagonal shape or any a polygonal shape.

The shape of the one or more of the edge surfaces (e.g., as viewed from the side) can be rectilinear or curvilinear in certain implementations. The shape may be regular or irregular in certain implementations. Similarly, the cross-section through the dielectric region 30 a and/or 30 b parallel to one of the major surfaces 31 a and 31 b, can be rectilinear or curvilinear in certain implementations and can be regular or irregular in certain implementations. For example, the cross-section can have a square shape, a rectangular shape, a circular shape, an oval shape, an elliptical shape, pentagonal shape, a hexagonal shape, an octagonal shape or any a polygonal shape. Other shapes are also possible. Likewise, the cross-section through the dielectric material or region 30 a and/or 30 b perpendicular to one of the surfaces 31 a and 31 b, can be rectilinear or curvilinear in certain implementations and can be regular or irregular implementations. For example, the cross-section can have a square shape, a rectangular shape, a circular shape, an oval shape, an elliptical shape, pentagonal shape, a hexagonal shape, an octagonal shape or any a polygonal shape. Other shapes are also possible. In various implementations, an area, a length and/or a width of the major surfaces 31 a and 31 b of the dielectric region 30 a can be greater than or equal to about 2, 3, 4, 5, 6, 8, or 10 times the thickness of the dielectric region 30 a and less than or equal to about 50 times the thickness of the dielectric region 30 a, or any value in a range/sub-range between any of these values. Accordingly, the dielectric region 30 a can have a large aspect ratio.

In some implementations, a thickness (T) of the dielectric region 30 a can correspond to the distance between the two major surfaces 31 a and 31 b along a vertical direction as shown in FIG. 9A. As another example, as shown in FIG. 9B, the dielectric material 30 b can be configured as a sphere. A thickness of the dielectric material 30 b configured as a sphere can correspond to the diameter of the sphere. In other implementations, the dielectric material 30 a and/or 30 b can be configured as a cube, a rectangular cuboid, a cylinder, an ellipsoid, an ovoid or any other three-dimensional shape. The shape may be curvilinear or rectilinear in certain implementations. The shape may be regular or irregular in certain implementations. Accordingly, in some implementations, the dielectric region 30 a and/or 30 b can be configured as an irregularly shaped object enclosing a volume of one or more dielectric materials.

In various implementations, light can be transmitted through the optical structure 70 a or 70 b and reflected by surfaces of the optical structure 70 a or 70 b. Moreover, in various implementations, the dielectric region 30 a and/or 30 b can have a thickness that allows light incident on one side of the metal layer 35 a and/or 35 b to constructively or destructively interfere. For example, in various implementations, the thickness of the dielectric region 30 a and/or 30 b can be approximately a quarter wavelength of light (e.g., visible light) incident thereon or an integer multiple of a quarter wavelength. In various implementations, the thickness of the dielectric region 30 a and/or 30 b may be, for example, 1/4, 3/4, 5/4, 7/4, 9/4, 10/4, etc. of the wavelength of visible light incident on the dielectric material 30 a or 30 b. As a result various wavelengths of incident light can constructively or destructively interfere as it is transmitted through the optical structure 70 a or 70 b or reflected by the optical structure 70 a or 70 b. Accordingly, in some configurations, color light is reflected by and/or transmitted through the optical structure when white light is incident thereon. In some implementations, a first color is reflected and a second different color is transmitted when white light is incident on the optical structure. In some case, the first color and the second color can be complementary.

In various implementations, for example, to obtain constructive interference of incident visible light, a thickness (or lateral dimension) of the dielectric region 30 a and/or 30 b can have a value between about 90 nm and about 2 microns. In various implementations, a thickness (or lateral dimension) of the dielectric region 30 a and/or 30 b can be greater than or equal to about 90 nm and less than or equal to about 1 microns, greater than or equal to about 100 nm and less than or equal to about 1.0 microns, greater than or equal to about 300 nm and less than or equal to about 1.0 microns, greater than or equal to about 400 nm and less than or equal to about 900 nm, greater than or equal to about 500 nm and less than or equal to about 800 nm, greater than or equal to about 600 nm and less than or equal to about 700 nm, or any thickness in any range/sub-range defined by these values. Values outside these ranges are also possible, in some implementations.

The dielectric material 30 a and/or 30 b can be purchased from various suppliers (e.g., Tyndall Institute, Glassflake, Ltd., Sigma Technologies) or custom made by synthesizing in a laboratory or a manufacturing facility. In some implementations, the optical structure 70 a (or 70 b) and/or the dielectric region 30 a (or 30 b) can comprise flakes (e.g., glass flakes available from Glassflake. Ltd. http://www.glassflake.com/pages/home). In some implementations, the flakes can comprise glass such as, for example, borosilicate flakes having an average thickness between about 90 nm and about 2 microns (e.g., an average thickness of about 1.2 microns) that may or may not be coated with coatings (e.g., high refractive index metal oxides such as TiO₂ and/or silica). In various implementations, lateral dimensions (e.g., length and a width) of the flakes can be between about 5 microns and about 20 microns. Values outside these ranges are also possible, in some implementations.

As discussed above, the dielectric region 30 a or 30 b can be surrounded by a partially reflective and a partially transmissive metal layer 35 a or 35 b. In some implementations, the metal layer 35 a or 35 b can comprise a metal having a ratio of the real part (n) of the refractive index to the imaginary part (k) of the refractive index (k) that is less than 1 as discussed above. For example, the metal layer 35 a or 35 b can comprise metals that have an n/k value between about 0.01 and about 0.6, between about 0.015 and about 0.6, between about 0.01 and about 0.5, between about 0.01 and about 0.2, between about 0.01 and about 0.1, or any value in a range or sub-range defined by these values. Values outside these ranges are also possible, in some implementations. Accordingly, the metal layer 35 a or 35 b can comprise silver, silver alloys, gold, aluminum or copper and their respective alloys, nickel (Ni) and palladium (Pd).

In various implementations, a thickness of the metal layer 35 a or 35 b can be configured such that the metal layer 35 a or 35 b is at least partially transmissive and partially reflective to light in the visible spectral region between about 400 nm and about 800 nm. For example, the thickness of the metal layer 35 can be configured such that the metal layer 35 a or 35 b is at least partially transmissive to light in a wavelength range between about 400 nm and about 500 nm, between about 430 nm and about 520 nm, between about 450 nm and about 530 nm, between about 520 nm and about 550 nm, between about 540 nm and about 580 nm, between about 550 nm and about 600 nm, between about 600 nm and about 680 nm, between about 630 nm and about 750 nm, or any wavelength in a range/sub-range defined by any of these values. Values outside these ranges are also possible, in some implementations. Alternatively or in addition, the thickness of the metal layer 35 a or 35 b can be configured such that the metal layer 35 a or 35 b is at least partially reflective to light in a wavelength range between about 400 nm and about 500 nm, between about 430 nm and about 520 nm, between about 450 nm and about 530 nm, between about 520 nm and about 550 nm, between about 540 nm and about 580 nm, between about 550 nm and about 600 nm, between about 600 nm and about 680 nm, between about 630 nm and about 750 nm, or any wavelength in a range/sub-range defined by any of these values. Values outside these ranges are also possible, in some implementations.

The thickness of the metal layer 35 a or 35 b can vary depending on the type of metal. For example, in implementations of the optical structure 70 a or 70 b comprising a metal (e.g., silver) layer 35 a or 35 b, the thickness of the metal (e.g., silver) layer 35 a or 35 b can be greater than or equal to about 10 nm and less than or equal to about 35 nm such that the metal (e.g., silver) layer 35 a or 35 b can be partially transmissive to light in the visible spectral range. In some implementations, the thickness of the metal layer 35 a or 35 b can be less than about 10 nm or greater than about 35 nm depending possibly on the type of metal used and the wavelength range in which transmissivity or transmittance is desired. Accordingly, in various implementations, the metal layer 35 a or 35 b can have a thickness greater than or equal to about 3 nm and less than or equal to about 40 nm. Values outside these ranges are also possible, in some implementations. As discussed above, with reference to FIG. 4 , the thickness of the metal layer 35 a or 35 b and the dielectric region 30 a or 30 b can be configured such that interference of some or all of the incident light reflected by the metal layer 35 a or 35 b and the one or more layers of the dielectric region 30 a or 30 b can produce a node at or in the metal layer 35 a or 35 b. Accordingly, the transmittance through the metal layer 35 a or 35 b can be greater than the transmittance expected for a certain thickness of the metal layer 35 a or 35 b. Without subscribing to any particular scientific theory, this effect is known as induced transmittance. As a result of induced transmittance or induced transmission, the optical structure 70 a or 70 b may in some implementation, be configured to exhibit a first color in reflection mode and a second color in transmission mode.

Depending on the shape of the dielectric region 30 a or 30 b, the dielectric region 30 a or 30 b can have one or more outer surfaces. The metal layer 35 a or 35 b can cover or substantially cover all the outer surfaces of the dielectric region 30 a or 30 b or a fraction thereof. Accordingly, in various implementations, the metal layer 35 a or 35 b can be disposed over at least 50% of the one or more outer surfaces of the dielectric region 30 a or 30 b. For example, metal layer 35 a or 35 b can be disposed over at least 50%, over at least 60%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 99%, 100%, or any range between any of these values of the one or more outer surfaces of the dielectric region 30 a or 30 b. In some implementations, the metal layer 35 a or 35 b can be disposed over the entire area (e.g., 100%) of the one or more outer surfaces of the dielectric region 30 a or 30 b. Without subscribing to any particular theory, the optical properties of the optical structure 70 a or 70 b can vary based on the amount of outer surface of the dielectric region 30 a or 30 b that is covered by the metal layer 35 a or 35 b. For example, the reflectivity or reflectance and/or the transmissivity or transmittance of the optical structure 70 a or 70 b can vary based on the amount of outer surface of the dielectric region 30 a or 30 b that is covered by the metal layer 35 a or 35 b.

In various implementations, the shape of the metal layer 35 a or 35 b can conform to the shape of the underlying dielectric material 30 a or 30 b. For example, in the optical structure 70 a shown in FIG. 9A, the dielectric material 30 a has a rectangular cross-section. Accordingly, the metal layer 35 a which is disposed over the major surfaces 31 a and 31 b and the edge surfaces also has a rectangular cross-section. As another example, in the optical structure 70 b shown in FIG. 9B, the dielectric material 30 b has a circular cross-section. Accordingly, the metal layer 35 b which is disposed over the circumference of the dielectric material 30 b also has a circular cross-section. However, in other implementations, the shape of the metal layer 35 a or 35 b can be different from the shape of the underlying dielectric material 30 a or 30 b.

In various implementations, the optical structure 70 a or 70 b comprising a dielectric region 30 a or 30 b surrounded by a metal layer 35 a or 35 b can be configured as particles, slabs, filaments, flakes, beads (e.g., spherical beads) or platelets as discussed above. In some implementations, the optical structure 70 a or 70 b comprising a dielectric region 30 a or 30 b surrounded by a metal layer 35 a or 35 b can have the same shape as the shape of the dielectric region 30 a or 30 b. For example, the optical structure 70 a can be configured as a cube or a rectangular cuboid when the dielectric region 30 a is configured as a cube or a rectangular cuboid as shown in FIG. 9A. As another example, the optical structure 70 b can be configured as a sphere when the dielectric region 30 b is configured as a sphere as shown in FIG. 9B. In some cases, the optical structure 70 a or 70 b configured as a particle, a slab, a flake, a filament, or a platelet can be suitable for a pigment or a printing ink. In some implementations, the optical structure 70 a or 70 b configured as a particle, a slab, a flake, a filament, or a platelet can have an area (or a lateral dimension) that is about 5 to 10 times or more the thickness of the optical structure 70 a or 70 b configured as a particle, a slab, a flake, a filament, or a platelet. Accordingly, an optical structure 70 a or 70 b configured as a particle, a slab, a flake, a filament, or a platelet can have a thickness between about 100 nm and about 1 micron. In some such implementations, the area (or a lateral dimension) can be greater than or equal to about 500 nm and less than or equal to about 1 micron, greater than or equal to about 1 micron and less than or equal to about 5 microns, greater than or equal to about 5 microns and less than or equal to about 10 microns, greater than or equal to about 5 micron and less than or equal to about 40 microns, greater than or equal to about 5 microns and less than or equal to about 20 microns, or any value in the ranges/sub-ranges defined by these values. In various embodiments, the optical structure 70 a or 70 b configured as a particle, a slab, a flake, a filament, or a platelet can be configured such that an area, a length and/or a width of a major surface of the optical structure 70 a or 70 b is greater than or equal to about 2, 3, 4, 5, 6, 8, or 10 times the thickness of the optical structure 70 a or 70 b and less than or equal to about 50 times the thickness of the optical structure 70 a or 70 b or any value in any range formed by any of these values.

In various implementations, surrounding the dielectric region 30 a or 30 b with the metal layer 35 a or 35 b can advantageously increase the reflectivity or reflectance of the dielectric material 30 a or 30 b at one or more wavelengths of the visible spectral range in some implementations. In some implementations, surrounding the dielectric material 30 a or 30 b with the metal layer 35 a or 35 b can advantageously enhance or change the color appearance of the dielectric material 30 a or 30 b at one or more wavelengths of the visible spectral range in reflection and transmission mode.

In various implementations, the optical structure 70 a or 70 b comprising the dielectric region 30 a or 30 b surrounded by the metal layer 35 a or 35 b can have a reflection spectrum with one or more reflection peaks in the visible spectral region and a transmission spectrum with one or more transmission peaks in the visible spectral region. Without any loss of generality, the one or more reflection peaks and the one or more transmission peak do not overlap with each other. Accordingly, the optical structure 70 a or 70 b comprising the dielectric region 30 a or 30 b surrounded by the metal layer 35 a or 35 b can have a first color in the reflection mode and a second color different from the first color in the transmission mode. In certain implementations, the first color and the second color can be complementary colors, such as, for example, red and green, yellow and violet, blue and orange, green and magenta, etc.

In various implementations, there may be little to no shift in the first color in the reflection mode for any viewing angle between a first angle with respect to a normal to the surface of the optical structure 70 a or 70 b and a second angle with respect to a normal to the surface of the optical structure 70 a or 70 b. Likewise, in some implementations, there may be little to no shift in the second color in the transmission mode for any viewing angle between a first angle with respect to a normal to the surface of the optical structure 70 a or 70 b and a second angle with respect to a normal to the surface of the optical structure 70 a or 70 b. In various implementations, the first angle can have a value between 0 degrees and 10 degrees (e.g., 0 degrees, 1 degree, 2 degrees, 3 degrees, 4 degrees, 5 degrees, 6 degrees, 7 degrees, 8 degrees, 9 degrees or 10 degrees). In various implementations, the second angle can have a value between 20 degrees and 90 degrees (e.g., 20 degrees, 30 degrees, 40 degrees, 50 degrees, 60 degrees, 70 degrees, 80 degrees or 90 degrees). Accordingly, for any viewing angle between a first angle (e.g., 0 degrees, 1 degree, 2 degrees, 3 degrees, 4 degrees, 5 degrees, 6 degrees, 7 degrees, 8 degrees, 9 degrees or 10 degrees) with respect to a normal to the surface of the optical structure 70 a or 70 b and a second angle (e.g., 20 degrees, 30 degrees, 40 degrees, 50 degrees, 60 degrees, 70 degrees, 80 degrees or 90 degrees) with respect to a normal to the surface of the optical structure 70 a or 70 b, the color of the optical structure 70 a or 70 b in the reflection mode and/or the transmission mode may remain substantially the same. Likewise, in some implementations, there may be little to no shift color shift in the color of the optical structure 70 a or 70 b in the reflection mode and/or the transmission mode for tilt of 10 degrees, 20 degrees, 30 degrees, 40 degrees, 50 degrees, 60 degrees, 70 degrees, 80 degrees or 90 degrees or any value in a range/sub-range defined by any of these values.

In some implementations, it may be desirable to have a color shift in the first color in the reflection mode as the viewing angle changes from a rust angle with respect to a normal to the surface of the optical structure 70 a or 70 b to a second angle with respect to a normal to the surface of the optical structure 70 a or 70 b. Similarly, in various implementations, it may be desirable to have a color shift in the second color in the transmission mode as the viewing angle changes from a first angle with respect to a normal to the surface of the optical structure 70 a or 70 b to a second angle with respect to a normal to the surface of the optical structure 70 a or 70 b. In various implementations, the first angle can have a value between 0 degrees and 10 degrees (e.g., 0 degrees, 1 degree, 2 degrees, 3 degrees, 4 degrees, 5 degrees, 6 degrees, 7 degrees, 8 degrees, 9 degrees or 10 degrees). In various implementations, the second angle can have a value between 20 degrees and 90 degrees (e.g., 20 degrees, 30 degrees, 40 degrees, 50 degrees, 60 degrees, 70 degrees, 80 degrees or 90 degrees) depending on the design. Accordingly, as the viewing angle changes from a first angle (e.g., 0 degrees, 1 degree, 2 degrees, 3 degrees, 4 degrees, 5 degrees, 6 degrees, 7 degrees, 8 degrees, 9 degrees or 10 degrees) with respect to a normal to the surface of the optical structure 70 a or 70 b to a second angle with respect to a normal to the surface of the optical structure 70 a or 70 b and a second angle (e.g., 20 degrees, 30 degrees, 40 degrees, 50 degrees, 60 degrees, 70 degrees, 80 degrees or 90 degrees) with respect to a normal to the surface of the optical structure 70 a or 70 b, the color of the optical structure 70 a or 70 b in the reflection mode and/or the transmission mode may change (e.g., dark blue to light blue, purple to pink, dark green to light green, etc.). Likewise, in some implementations, there may be a shift in the color of the optical structure 70 a or 70 b in the reflection mode and/or the transmission mode for tilt of 10 degrees, 20 degrees, 30 degrees, 40 degrees, 50 degrees, 60 degrees, 70 degrees, 80 degrees or 90 degrees or any value in a range/sub-range defined by any of these values.

Without subscribing to any particular theory, the one or more reflection peaks of the reflection spectrum of the optical structure 70 a or 70 b comprising the dielectric region 30 a or 30 b surrounded by the metal layer 35 a or 35 b can have high reflectivity or reflectance. For example, the reflectivity or reflectance of the one or more reflection peaks can be greater than or equal to 30%, greater than or equal to 40%, greater than or equal to 50%, greater than or equal to 55%, greater than or equal to 60%, greater than or equal to 65%, greater than or equal to 70%, greater than or equal to 75%, greater than or equal to 80%, greater than or equal to 85%, greater than or equal to 90%, greater than or equal to 95% and less than or equal to 100%, or a value in any range/sub-range defined by these values.

Without subscribing to any particular theory, the one or more transmission peaks of the transmission spectrum of the optical structure 70 a or 70 b comprising the dielectric region 30 a or 30 b surrounded by the metal layer 35 a or 35 b can have high transmissivity or transmittance. For example, the transmissivity or transmittance of the one or more transmission peaks can be greater than or equal to 30%, greater than or equal to 40%, greater than or equal to 50%, greater than or equal to 55%, greater than or equal to 60%, greater than or equal to 65%, greater than or equal to 70%, greater than or equal to 75%, greater than or equal to 80%, greater than or equal to 85%, greater than or equal to 90%, greater than or equal to 95% and less than or equal to 100%, or a value in any range/sub-range defined by these values.

The optical structures 70 a and 70 b comprising the dielectric region 30 a or 30 b surrounded by the metal layer 35 a or 35 b can produce many or all the optical effects that are described above with reference to optical structure 10 where the two metal layers 13 and 15 do not surround the dielectric layer 14 (e.g., as shown in FIG. 1 ).

The metal layer 35 a or 35 b can be disposed around the dielectric material 30 a or 30 b using a variety of chemical methods. For example, metal layer 35 a or 35 b can be disposed around the dielectric region 30 a or 30 b using electroless method. Various implementations of an electroless method of depositing the metal layer 35 a or 35 b can comprise depositing the metal layer 35 a or 35 b without applying electrical current or voltage. Various metals such as, for example, gold, silver, or nickel can be deposited using electroless methods. An example of depositing metal layer 35 a or 35 b comprising silver around the dielectric region 30 a or 30 b using an electroless method is discussed below. The electroless method of depositing silver can also be referred to as electroless silver plating. Electroless silver plating comprises immersing the dielectric region 30 a or 30 b in a silvering bath comprising chemical compounds of silver (e.g., silver nitrate, silver-ammonia compounds, sodium argento cyanide, etc.) and at least one of ammonia, water, potassium hydroxide or sodium hydroxide. The chemical compounds of silver are reduced to metallic silver using a reducing agent which is added to the silvering bath. The metallic silver adheres to the exposed surfaces of the dielectric region 30 a or 30 b. The reducing agent can comprise glucose, sucrose, invert sugar, stannous chloride, hydrazine, Rochelle salt, formaldehyde, or organic borane (e.g., dimethylamine borane in various implementations). In certain implementations, the silvering bath and the reducing agent can be sprayed on the dielectric region 30 a or 30 b. In some implementations, the outer surface of the dielectric region 30 a or 30 b can be activated using stannous chloride (SnCl₂) in preparation for the electroless deposition of the metal layer. Other methods of depositing the metal layer 35 a or 35 b on the outer surface of the dielectric region 30 a or 30 b can also be used. For example, the metal layer 35 a or 35 b can be disposed around the dielectric region 30 a or 30 b using methods such as, for example, chemical vapor deposition (CVD), sputtering or electroplating. In some implementations, the metal layer 35 a or 35 b can be patterned around the dielectric region 30 a or 30 b.

In various implementation, a second dielectric region 40 a or 40 b comprising one or more dielectric materials may be disposed around the metal coated dielectric region 30 a or 30 b. The second dielectric region 40 a or 40 b may comprise high refractive index materials such as ZrO₂, TiO₂, ZnS, ITO (indium tin oxide), CeO₂ or Ta₂O₃. In various implementations, the second dielectric region 40 a or 40 b may comprise dielectric materials having refractive index greater than 1.65 and less than or equal to 2.5. For example, the refractive index of the one or more dielectric material in the second dielectric region 40 a or 40 b can be greater than or equal to 1.65 and less than or equal to 1.75, greater than or equal to 1.75 and less than or equal to 1.85, greater than or equal to 1.85 and less than or equal to 1.95, greater than or equal to 1.95 and less than or equal to 2.05, greater than or equal to 2.0 and less than or equal to 2.2, greater than or equal to 2.1 and less than or equal to 2.3, greater than or equal to 2.25 and less than or equal to 2.5, or any value in any range/sub-range defined by these values. Other values outside these ranges are also possible in some implementations. In various implementations, the refractive index of the one or more materials of the second dielectric region 40 a or 40 b can be greater than the refractive index of the one or more materials of the dielectric region 30 a or 30 b. The thickness of the second dielectric region 40 a or 40 b can be between 75 nm and 700 nm. For example, the thickness of the second dielectric region 40 a or 40 b can be greater than or equal to 75 nm and less than or equal to 100 nm, greater than or equal to 100 nm and less than or equal to 150 nm, greater than or equal to 150 nm and less than or equal to 200 nm, greater than or equal to 200 nm and less than or equal to 250 nm, greater than or equal to 300 nm and less than or equal to 350 nm, greater than or equal to 400 nm and less than or equal to 450 nm, greater than or equal to 450 nm and less than or equal to 500 nm, greater than or equal to about 500 nm and less than or equal to 650 nm, greater than or equal to 650 nm and less than or equal to 700 nm, or any value in any range/sub-range defined by these values. The second dielectric region 40 a or 40 b can be disposed to cover at least 50% of the outer surface of the metal layer 35 a or 35 b. For example, the second dielectric region 40 a or 40 b can be disposed to cover at least 80%, at least 90%, at least 95%, or 100% of the outer surface of the metal layer 35 a or 35 b, or any value in a range/sub-range defined by these values.

The reflected color and/or the transmitted color of the optical structure 70 a or 70 b comprising the second dielectric region 40 a or 40 b surrounding the metal coated dielectric region 30 a or 30 b can be different from the reflected color and/or the transmitted color of the optical structure 70 a or 70 b comprising only the metal coated dielectric region 30 a or 30 b. For example, the reflected color and/or the transmitted color of the optical structure 70 a or 70 b comprising the second dielectric region 40 a or 40 b surrounding the metal coated dielectric region 30 a or 30 b can be more vibrant than the reflected color and/or the transmitted color of the optical structure 70 a or 70 b comprising the metal coated dielectric region 30 a or 30 b without the second dielectric region 40 a or 40 b having suitable thickness and/or materials with suitable refractive index. The shape of the transmission and/or reflection peaks, the position of the maximum of the transmission and/or reflection peaks and/or the width (e.g., full width at half maximum (FWHM)) of the transmission and/or reflection peaks of the optical structure 70 a or 70 b comprising the second dielectric region 40 a or 40 b surrounding the metal coated dielectric region 30 a or 30 b can be different from the shape of the transmission and/or reflection peaks, the position of the maximum of the transmission and/or reflection peaks and/or the width of the transmission and reflection peaks of the optical structure 70 a or 70 b comprising the metal coated dielectric region 30 a or 30 b without the second dielectric region 40 a or 40 b having suitable thickness and/or materials with suitable refractive index. For example, the width of one or more of the reflection peaks of the optical structure 70 a or 70 b comprising the second dielectric region 40 a or 40 b surrounding the metal coated dielectric region 30 a or 30 b can be broader than the width of a corresponding reflection peak of the optical structure 70 a or 70 b comprising the metal coated dielectric region 30 a or 30 b without the second dielectric region 40 a or 40 b having suitable thickness and/or materials with suitable refractive index. As another example, the width (e.g., FWHM) of one or more of the reflection peaks of the optical structure 70 a or 70 b comprising the second dielectric region 40 a or 40 b surrounding the metal coated dielectric region 30 a or 30 b can be greater than or equal to about 50 nm and less than or equal to about 300 nm, in some implementations.

Various implementations of the of the optical structure 70 a or 70 b comprising the second dielectric region 40 a or 40 b surrounding the metal coated dielectric region 30 a or 30 b can have a reflection spectrum with one or more reflection peaks having a width (e.g., FWHM) greater than or equal to about 10 nm, greater than or equal to about 20 nm, greater than or equal to about 30 nm, greater than or equal to about 40 nm, greater than or equal to about 50 nm, greater than or equal to about 60 nm, greater than or equal to about 70 nm, greater than or equal to about 100 nm, greater than or equal to about 200 nm, less than or equal to about 300 nm, less than or equal to about 250 nm, or any value in a range/sub-range defined by these values. Various implementations of the optical structure 70 a or 70 b comprising the second dielectric region 40 a or 40 b surrounding the metal coated dielectric region 30 a or 30 b can have higher reflectivity or reflectance at one or more wavelengths in the visible spectral range as compared to the reflectivity or reflectance of the optical structure 70 a or 70 b comprising the metal coated dielectric region 30 a or 30 b without the second dielectric region 40 a or 40 b having suitable thickness and/or materials with suitable refractive index at those one or more wavelengths in the visible spectral range.

Various implementations of the of the optical structure 70 a or 70 b comprising the second dielectric region 40 a or 40 b surrounding the metal coated dielectric region 30 a or 30 b can have a transmission spectrum with one or more transmission peaks having a width (e.g., FWHM) greater than or equal to about 10 nm, greater than or equal to about 20 nm, greater than or equal to about 30 nm, greater than or equal to about 40 nm, greater than or equal to about 50 nm, greater than or equal to about 60 nm, greater than or equal to about 70 nm, greater than or equal to about 100 nm, greater than or equal to about 200 nm, less than or equal to about 300 nm, less than or equal to about 250 nm, or any value in a range/sub-range defined by these values.

Without subscribing to any particular theory, the one or more reflection peaks of the reflection spectrum of the optical structure 70 a or 70 b comprising the second dielectric region 40 a or 40 b surrounding the metal coated dielectric region 30 a or 30 b can have high reflectivity or reflectance. For example, the reflectivity or reflectance of the one or more reflection peaks can be greater than or equal to 30%, greater than or equal to 40%, greater than or equal to 50%, greater than or equal to 55%, greater than or equal to 60%, greater than or equal to 65%, greater than or equal to 70%, greater than or equal to 75%, greater than or equal to 80%, greater than or equal to 85%, greater than or equal to 90%, greater than or equal to 95% and less than or equal to 100%, or a value in any range/sub-range defined by these values.

Without subscribing to any particular theory, the one or more transmission peaks of the transmission spectrum of the optical structure 70 a or 70 b comprising the second dielectric region 40 a or 40 b surrounding the metal coated dielectric region 30 a or 30 b can have high transmissivity or transmittance. For example, the transmissivity or transmittance of the one or more transmission peaks can be greater than or equal to 30%, greater than or equal to 40%, greater than or equal to 50%, greater than or equal to 55%, greater than or equal to 60%, greater than or equal to 65%, greater than or equal to 70%, greater than or equal to 75%, greater than or equal to 80%, greater than or equal to 85%, greater than or equal to 90%, greater than or equal to 95% and less than or equal to 100%, or a value in any range/sub-range defined by these values.

Additionally, the second dielectric region 40 a or 40 b can advantageously insulate the metal layer 35 a or 35 b from the ink varnish when the optical structures 70 a or 70 b are configured as pigments.

In some implementations, the second dielectric region 40 a or 40 b can be disposed around the metal coated dielectric materials 30 a or 30 b using a sol-gel process. For example, the metal coated dielectric materials 30 a or 30 b can be coated with a dielectric material comprising titanium di-oxide (TiO₂) using a sol-gel process, involving the hydrolysis of titanium(IV) isopropoxide. As another example, a precursor comprising the dielectric material 40 a or 40 b is transformed to form a colloidal suspension (or a “sol”) by a series of hydrolysis and polymerization reactions. In some implementations, the colloidal suspension comprising the dielectric material of the second dielectric region 40 a or 40 b can be disposed on the metal coated first dielectric region 30 a or 30 b by a coating, gelling or precipitation. The metal coated first dielectric region 30 a or 30 b comprising the colloidal suspension comprising the dielectric material of the second dielectric region 40 a or 40 b can be heated or dried to obtain the metal coated first dielectric region 30 a or 30 b coated with second dielectric region 40 a or 40 b. In some implementations, the one or more materials of the second dielectric region 40 a or 40 b can be disposed around the metal coated first dielectric region 30 a or 30 b using deposition methods such as, for example, chemical vapor deposition method, e-beam, sputtering. In some implementations, the various deposition methods can be combined with vibrating the metal coated first dielectric region 30 a or 30 b.

As discussed above, various embodiments of the optical structures 10, 70 a or 70 b are configured to partially reflect light and partially transmit light. In various implementations, the reflectivity or reflectance of the optical structures 10, 70 a or 70 b at one or more wavelengths in the visible spectral range can be greater than or equal to 10%, greater than or equal to 20%, greater than or equal to 30%, greater than or equal to 40%, greater than or equal to 50%, greater than or equal to 60%, greater than or equal to 70%, greater than or equal to 80%, greater than or equal to 90%, greater than or equal to 95% and/or less than or equal to 100%, or any value in any range/sub-range defined by these value. In various implementations, the transmissivity or transmittance of the optical structures 10, 70 a or 70 b at one or more wavelengths in the visible spectral range can be greater than or equal to 10%, greater than or equal to 20%, greater than or equal to 30%, greater than or equal to 40%, greater than or equal to 50%, greater than or equal to 60%, greater than or equal to 70%, greater than or equal to 80%, greater than or equal to 90%, greater than or equal to 95% and/or less than or equal to 100%, or any value in any range/sub-range defined by these value. In various implementations, the reflectivity or reflectance of the optical structures 10, 70 a or 70 b at one or more first set of wavelengths can be approximately equal to the transmissivity or transmittance of the optical structures 10, 70 a or 70 b at one or more second set of wavelengths different from the first set of wavelengths.

The optical structures 10, 70 a or 70 b can have a size, such as, for example, a lateral dimension, an area, a length or a width of the optical structure (e.g., a length, a width or an area of a major surface of the optical structure) greater than or equal to about 1 micron and less than or equal to about 50 microns. For example, the size of the optical structures 10, 70 a or 70 b can be greater than or equal to about 1 micron and less than or equal to 10 microns, greater than or equal to 2 microns and less than or equal to 12 microns, greater than or equal to 3 microns and less than or equal to 15 microns, greater than or equal to 4 microns and less than or equal to 18 microns, greater than or equal to 5 microns and less than or equal to 20 microns, greater than or equal to 10 microns and less than or equal to 20 microns, greater than or equal to 15 microns and less than or equal to 25 microns, greater than or equal to 20 microns and less than or equal to about 30 microns, greater than or equal to 25 microns and less than or equal to 35 microns, greater than or equal to 30 microns and less than or equal to 40 microns, greater than or equal to 35 microns and less than or equal to 45 microns, greater than or equal to 40 microns and less than or equal to 50 microns, or a value in any range/sub-range defined by these values.

The optical structures 10, 70 a or 70 b can have a size, such as, for example, a lateral dimension, an area, a length or a width of the optical structure (e.g., a length, a width or an area of a major surface of the optical structure) greater than or equal to about 1 micron and less than or equal to about 50 microns can be between 0.1 microns and 2.0 microns. For example, the thickness of the optical structures 10, 70 a or 70 b having a size, such as, for example, a lateral dimension, an area, a length or a width of the optical structure (e.g., a length, a width or an area of a major surface of the optical structure) greater than or equal to 0.1 micron and less than or equal to 0.3 microns, greater than or equal to 0.2 microns and less than or equal to 0.5 microns, greater than or equal to 0.3 microns and less than or equal to 0.6 microns, greater than or equal to 0.4 microns and less than or equal to 0.7 microns, greater than or equal to 0.5 microns and less than or equal to 0.8 microns, greater than or equal to 0.6 microns and less than or equal to 0.9 microns, greater than or equal to 0.7 microns and less than or equal to 1.0 micron, greater than or equal to 1.0 micron and less than or equal to 1.2 microns, greater than or equal to 1.2 microns and less than or equal to 1.5 microns, greater than or equal to 1.5 microns and less than or equal to 2.0 microns, or a value in any range/sub-range defined by these values.

One or more of the optical structures 10, 70 a or 70 b discussed above can be incorporated with or in a document (e.g., a banknote), package, product, or other item. Optical products such as a film, a thread, a laminate, a foil, a pigment, or an ink comprising one or more of the optical structures 10, 70 a or 70 b discussed above can be incorporated with or in documents such as banknotes or other documents to verify authenticity of the documents, packaging materials, etc. For example, the optical structures 70 a or 70 b can be configured as an ink or a pigment which is disposed on a base comprising at least one of a polymer, a plastic, a paper or a fabric. The base may be flexible in some implementations. The base comprising the ink or a pigment or pigment comprising the optical structures 70 a or 70 b can be cut or diced to obtain a thread or a foil. A plurality of optical structures 10, 70 a or 70 b discussed above can be incorporated in a particular optical product (e.g., ink, pigment, thread, filament, paper, security ink, security pigment, security thread, security filament, security paper, etc.). The shapes, sizes and/or aspect ratios of the plurality of optical structures 10, 70 a or 70 b discussed above that are incorporated in a particular optical product (e.g., ink, pigment, thread, filament, paper, security ink, security pigment, security thread, security filament, security paper, etc.) can vary. Accordingly, a particular optical product (e.g., ink, pigment, thread, filament, paper, security ink, security pigment, security thread, security filament, security paper, etc.) can comprise optical structures 10, 70 a or 70 b with different distributions of shapes, sizes and/or aspect ratios of the optical structures. For example, a particular optical product (e.g., ink, pigment, thread, filament, paper, security ink, security pigment, security thread, security filament, security paper, etc.) can comprise optical structures 10, 70 a or 70 b with sizes distributed around one or more mean sizes. As another example, a particular optical product (e.g., ink, pigment, thread, filament, paper, security ink, security pigment, security thread, security filament, security paper, etc.) can comprise optical structures 10, 70 a or 70 b with aspect ratios distributed around one or more aspect ratios.

FIG. 10 shows, for example, a banknote 80 comprising a laminated film 83. The laminated film 83 comprises the optical structure 10, 70 a or 70 b. The laminated film 83 can be fabricated by disposing the optical structure 10, 70 a or 70 b over a base or support layer or substrate such as polymer base layer (e.g., a polyester film). The optical structure 10, 70 a or 70 b can be disposed over the polymer base layer by a variety of methods including but not limited to coating methods, vacuum deposition on a surface of the polymer base layer, etc. The optical structure 10, 70 a or 70 b may be disposed over a first side of the surface of the polymer base layer (e.g., polyester film). The laminated film 83 can be adhered to the “paper” (e.g., cellulose, cotton/linen, polymer or fabric) 81 of the banknote 80, for example, by a transparent and/or an optically clear adhesive. In various cases, a second surface of the polymer base layer opposite the first surface of the base layer is disposed closer to the banknote paper 81 comprising the banknote and may be in contact with the adhesive. In some cases, the adhesive can be a two component adhesive with one component disposed onto the banknote paper and the other component disposed on the second surface of the polymer base layer opposite the first surface of the base layer on which the optical structure 10, 70 a or 70 b is disposed. The banknote 80 and the laminated film 83 can be brought together for bonding. The laminated film 83 can also be attached to the banknote 80 using a cross-linking thermoset adhesive. A transparent protective barrier coating 82 (e.g., UV curable cross-linked resin) can be disposed over the laminated film 83. The protective barrier coating 82 can extend over the edges of the laminated film 83 onto the paper (e.g., fabric) 81 of the banknote. The protective barrier coating 82 can be configured to protect the laminated film 83 against corrosion, abrasive wear and liquids that may commonly come in contact with the banknote 80 without sacrificing the optical effects provided by the laminated film 83. The optical structure 10 can be disposed facing the protective barrier coating 82 or the adhesive layer between the laminated film 83 and the “paper” 81.

In some embodiments, the optical structure 10, 70 a or 70 b can be configured as a thread (e.g., a windowed thread) instead of a laminated film. A windowed thread can be manufactured by a variety of methods. For example, the thread can be woven up and down within the paper and to the surface of the paper during the papermaking process. As another example, the windowed thread can be disposed within the paper itself so that no part of the thread reaches the surface of the banknote. As yet another example, open spaces within the paper can be provided in the regions of the paper comprising the thread.

The thread can be fabricated by cutting a strip of the optical structure 10, for example the web, sheet, or base layer on which the layers comprising the optical structure 10 are formed and passing the strip through a bath of UV curable resin. The rate at which the strip is passed through the UV curable resin bath can be controlled to coat the sides and the edges of the strip uniformly. The strip coated with the UV curable resin can be cured to obtain the thread. The obtained thread comprising the optical structure 10 can be inserted (e.g., weaved) in the banknote. In some implementations, any fringe (e.g., the jagged or ragged edge of the thread) of the thread (due to hot stamping or chatter from any cutting operation) can be hidden from an observer by printing an opaque border around the hot stamp patch. Another way to affix the optical structure 10, 70 a or 70 b to the banknote can include die cutting a portion of the optical structure, for example, the web, sheet, or base layer on which the layers comprising the optical structure 10, 70 a or 70 b are formed and applying the portion to the banknote using an adhesive. Various implementations of the examples of optical structure described above can be configured as a thread, a hot stamp, or a laminate and incorporated with or in a document (e.g., a banknote), package, product, or other item.

Without any loss of generality, the optical structure 10, 70 a or 70 b or a material (e.g., an ink, a paint or a pigment, a varnish) comprising the optical structure 10, 70 a or 70 b can be disposed on a base comprising at least one of a polymer, a plastic, a paper or a fabric. The base comprising the optical structure 10, 70 a or 70 b or the material comprising the optical structure 10, 70 a or 70 b can be cut or diced into a smaller portions having a variety of shapes and/or sizes. The smaller portions can be disposed on or inserted into or onto a substrate (e.g., a bank note, paper, packaging material, fabric, etc.) using various methods. For example, the smaller portions can be configured as strips or threads which can be woven into the substrate. As another example, the smaller portions can be configured as foils which can be hot stamped on the substrate. As yet another example, the smaller portions can be laminated to the substrate using adhesives.

FIG. 11A depicts a banknote 90 a having two transparent windows 91 a and 92 a inserted into or attached on the paper (e.g., fabric) of the banknote. Each window comprises the optical structure 10, 70 a or 70 b. In some implementations, the reflection and/or transmission spectra of the optical structure 10 of the window 91 a may be configured to be different from the reflection and/or transmission spectra of the optical structure 10, 70 a or 70 b of the window 92 a. Thus, a person viewing the banknote 90 a will perceive a first reflected color when viewing the window 91 a along a viewing direction (e.g., normal to the surface of the banknote 90 a) and a second reflected color different from the first reflected color when viewing the window 92 a along the viewing direction. The person may also perceive a third transmitted color different from the first reflected color when viewing through the window 91 a along the viewing direction. The person may additionally perceive a fourth transmitted color different from the first, second and third colors when viewing through the window 92 a along the viewing direction. Furthermore, upon folding the banknote 90 a over itself so that the two windows 91 a and 92 a are at least partially aligned with respect to one another, the person will perceive a different color, different from the first, second, third and/or fourth colors in reflection and transmission modes when viewing the banknote 90 a along the viewing direction. For example, upon folding the banknote 90 a over itself so that the two windows 91 a and 92 a are at least partially aligned with respect to one another, the person will perceive a reflected color that is a combination of the effects of the reflectivity or reflectance spectrums of the two windows 91 a and 92 a and a transmitted color that is a combination of the effects of the transmission spectrums of the two windows 91 a and 92 a. Additionally, the person can perceive color shift of the various colors seen in the reflection and transmission modes as the viewing angle changes. The amount of color shift may be different from the different windows as well as for the combination of the two windows.

FIG. 11B depicts an implementation of a security device 90 b (e.g., a banknote) comprising two windows 91 b and 92 b (a first and a second) inserted into or attached to the surface of the security device 90 b. The two windows 91 b and 92 b at least partially overlap in the overlapping region 93 b. The two windows 91 b and 92 b are transparent and comprise the optical structure 10, 70 a or 70 b. The configuration (e.g., thickness or other design parameters) of the optical structures 10, 70 a or 70 b in the respective windows 91 a and 91 b can be such that the reflection and/or transmission spectra of the optical structure 10, 70 a or 70 b of the window 91 b is different from the reflection and/or transmission spectra of the optical structure 10 of the window 92 b.

Thus, a person viewing the security device 90 b along a viewing direction (e.g., normal to the surface of the security device 90 b) will perceive (i) a first reflected color when viewing the portion of the window 91 b that does not overlap with the window 92 b, (ii) a second reflected color different from the first color when viewing the portion of the window 92 b that does not overlap with the window 91 b; and (iii) a third second reflected color that is a combination of the effects of the reflectivity or reflectance spectrums of the two windows 91 b and 92 b when viewing the overlapping region 93 b.

A person viewing the security device 90 b along a viewing direction (e.g., normal to the surface of the security device 90 b) will perceive (i) a fourth transmitted color different from the first color when viewing through the portion of the window 91 b that does not overlap with the window 92 b. (ii) a fifth transmitted color different from the second and the fourth color when viewing through the portion of the window 92 b that does not overlap with the window 91 b; and (iii) a sixth transmitted color that is a combination of the effects of the transmission spectrums of the two windows 91 b and 92 b when viewing through the overlapping region 93 b.

Additionally, in various embodiments, a person viewing the security device 90 b can perceive color shift of the various colors seen in the reflection and transmission modes as the viewing angle changes. The amount of color shift may be different from the different windows.

Although, the two windows 91 b and 92 b are shown as partially overlapping in FIG. 11B, the two windows 91 b and 92 b can be completely overlapping. Various implementations of the security device 90 b can comprise two or more different pigments. The two or more different pigments can comprise optical structures 10. A respective optical structure of one of the two or more different pigments can have reflectance and transmittance characteristics that are different from the respective optical structure of another of the two or more different pigments. The two or more different pigments can partially or completely overlap with each other. As discussed above, the color perceived by a person viewing an overlapping region of the two or more different pigments can depend on a combination of the effects of the reflection/transmission spectra of the different optical structures of the two or more different pigments. Some implementations of the security device 90 b can comprise two or more at least partially overlapping foils, films, threads or laminates comprising different optical structures. The color perceived by a person viewing an overlapping region of the two or more at least partially overlapping foils, films, threads or laminates can depend on a combination of the effects of the reflection/transmission spectra of the different optical structures of the two or more foils, films, threads or laminates.

FIG. 12 illustrates a side view of an object 100 with a security device comprising a main body 103 of the object and a layer 102 comprising the optical structure 10, 70 a or 70 b. The object can be a banknote. The main body may comprise paper comprising the banknote. The layer 102 can be a laminate, a thread, or a label. When the layer 102 is configured as a label, an adhesive (e.g., a varnish) can be applied to the main body 103 and the layer 102 can be adhered to the adhesive of the main body 103 using a polymeric adhesive. Alternatively, the adhesive can be applied to the layer 102 before being affixed to the main body 103. When the layer 102 is configured as a laminate, the layer 102 can be adhered to the main body 103 using a polymer.

The layer 102 can be adhered to the main body 103 using adhesives, such as, for example optical clear adhesive and/or a cross-linking thermoset adhesive. The security device 100 further comprises a layer 101 comprising a message that is composed using a text, a symbol, a number or any combination thereof that is disposed on the side of the main body (e.g., paper/fabric) 103 of the object (e.g., banknote) opposite the side on which the layer 102 as shown in FIG. 12 . Alternately, the layer 101 can be disposed between the main body (e.g., paper/fabric) 103 and the layer 102 or over the layer 102. The layer 101 can comprise, for example, a dye, a pigment or a phosphorescent material that has the same color characteristics as the color reflected or transmitted by the optical structure 10 when viewed along a direction normal to the surface of the layer 102. Accordingly, the message is not visible to an observer (or hidden) when the security device 100 is viewed along a direction normal to the surface of the layer 102. However, when the security device 100 is tilted such that viewing angle changes, the color reflected by and/or transmitted through the optical structure 10 changes such that the message become visible to the observer. In certain cases, the layer 101 comprising a message printed with a phosphorescent material can be made visible when illuminated by UV. The resultant color of the phosphorescent material can be the combined color of the fluoresence and the dichroic color.

FIG. 13 shows the effect of changing the viewing angle in transmission of the security device 100 from 0 to about 45 degrees. When the viewing angle is 0 degrees, the message comprising a combination of a number, text or a symbol is not visible in the transmission mode because the color of the text is the same as the color of the optical structure in transmission mode (e.g., orange). However, as the viewing angle increases, the color of the optical structure in transmission mode shifts. For example, the message 203 becomes visible as the color of the optical structure in transmission mode shifts from orange to yellow as the angle of observation increases. The color of the message has sufficient contrast with respect to the transmitted color of the optical structure 10 so as to be visible to the observer.

In other embodiments, the security device 100 can be configured to operate in reverse to that described above such that for example the message is visible at normal incidence and not visible when the security device is tilted. Other variations are possible.

As describe above, the optical structures 10, 70 a or 70 b may be used in different forms, such as a laminate, a foil, a film, a hot stamp, a thread, pigment, ink, or paint. In some implementations, a laminate, a foil, a film, or a thread can comprise a pigment, ink or paint comprising the optical structures 10, 70 a or 70 b. A laminate may be adhered to a document, product or package using adhesive. A thread may be threaded or woven through an opening, for example, in the document. A foil can be hot stamped on the document, product or package. Pigment, ink, or paint may be deposited on the document, product or package or the material (e.g., paper, cardboard, or fabric) used to form the document, product, or package. For example, the document, product, or package may be exposed to (e.g., contacted with) the pigment, ink, or paint to color the document, product or packages in process similar to those used for non-color shifting pigments, dyes, paints and inks.

A plurality of optical structures 10, 70 a or 70 b such as described herein collected together as a pigment (as well as inks, and paints) can have similar optical characteristics as the optical structure 10, 70 a or 70 b configured as a film/laminate. As described above, optical structures 10, 70 a or 70 b collected together to form a pigments can exhibit as a collection of platelets or separate pieces the same optical characteristics as the bulk optical film from which the platelets were made. An added advantage of the optical structures 10, 70 a or 70 b configured as a pigment is that color can be blended according to desired specification. The color of the optical structure 10 can be designed by using computer software to calculate the thickness of the various layers of the optical structure 10, 70 a or 70 b that would provide a desired reflection and/or transmission characteristics. Optical structures 10, 70 a or 70 b that can provide specific colors can be designed using the computer software and then fabricated. Additionally, different color shifting optical structures 10, 70 a or 70 b that produce different colors can be included together and/or color shifting optical structures such as described herein can be combined with non-color shifting pigments or dyes to produce different colors.

The optical structure 10, 70 a or 70 b can be fabricated using a variety of methods including but not limited to vacuum deposition, coating methods, etc. One method of fabrication of the optical structures 10 described herein uses a vacuum coater that employs batch or roll coating. In one method of fabricating the optical structure 10, a first transparent high index layer (e.g., layer 12 or layer 16 of FIG. 1 ) is deposited onto carrier or base layer such as a sheet or web or other substrate. The carrier, web, base layer or substrate can comprise materials such as, for example, polyester or a polyester with release characteristics such that the optical structure can be readily separated from the web or base layer. A release layer between the base layer and the plurality of other optical layers the form the optical structure may be used to permit separation of the optical layers comprising the optical structure from the base layer or web. A first metal layer (e.g., layer 13 or layer 15), a transparent dielectric layer comprising high or low refractive index material (e.g., layer 14), a second metal layer (e.g., layer 15 or layer 13), and a second transparent high index layer (e.g., layer 16 or layer 12) is deposited over the first transparent high index layer in sequence (e.g., layer 12 or layer 16 of FIG. 1 ). The various layers can be deposited in sequence in some embodiments. However, in other embodiments, one or more intervening layers can be disposed between any of the first metal layer, the transparent dielectric layer comprising high or low refractive index material, the second metal layer, and the second transparent high index layer. As examples, in some cases the transparent high index layers and the dielectric layer can be deposited using electron gun while the first and the second metal layers can be deposited by using electron gun or sputtering.

Some materials, like ZnS or MgF2, can be evaporated from a resistance source. In instances wherein the transparent dielectric layer comprising high or low refractive index material comprises a polymer, a process known as PML (Polymer Multi-Layer) as described in U.S. Pat. No. 5,877,895 can be used. The disclosure of U.S. Pat. No. 5,877,895 is incorporated by reference herein in its entirety.

The disclosure set forth herein describes a wide variety of structures and method but should not be considered to be limited to those particular structures or methods. For example, although many of the example optical structures 10 are symmetrical, asymmetric structures are also possible. For example, instead of having a pair of similar or identical dielectric layers sandwiching the pair of metallic layers, either dielectric layers having different characteristics (e.g., thickness or material) may be used on opposite sides of the structure or alternatively, maybe only one side of the pair of metal layers has a dielectric layer thereon. Similarly, the metal layers need not be identical and may have different characteristics such as different thicknesses or materials. As described above, intervening layers may also be included. One or more such intervening layer may be include such that the optical structure is not symmetric. For example, an intervening layer may be included between the dielectric layer 12 and metal layer 13 and not between that metal layer 15 and the dielectric layer 16 or vice versa. Similarly, an intervening layer may be included between the metal layer 13 and the dielectric layer 14 and not between the dielectric layer 14 and the metal layer 15, or vice versa. Similarly, intervening layer having different characteristics (e.g., material or thickness) may be included on different sides of the optical structure 10. Or more intervening layers may be include on one side of the optical structure 10 than on the other side of the optical structure. For example, the metal layer 13 and/or the metal layer 15 can comprise metal sub-layers. In some implementations, the metal layer 13 and/or the metal layer 15 can comprise a first metal (e.g., silver) facing the high refractive index transparent layers 12 or 16 and a second metal (e.g., gold) facing the dielectric layer 14.

Likewise, although this disclosure describes applications of the structures and method describe herein to include security applications, e.g., countering successful use of counterfeit currency, documents, and products, this disclosure should not be considered to be limited to those particular applications. Alternatively or in addition, such features could, for example, be used for providing an aesthetic effect, to create appealing or attractive features on products or packaging for marketing and advertisement, or for other reasons.

Dimensions, such as, thickness, length, width of various embodiments described herein can be outside the different ranges provided in this disclosure. The values of refractive indices for the various materials discussed herein can be outside the different ranges provided in this disclosure. The values for reflectance and/or transmittance of the different structures can be outside the different ranges provided herein. The values for spectral widths and peak locations for the reflection and transmission spectra can be outside the different ranges provided herein.

Various embodiments of the present invention have been described herein. Although this invention has been described with reference to these specific embodiments, the descriptions are intended to be illustrative of the invention and are not intended to be limiting. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention. 

What is claimed is:
 1. An optical structure comprising: a first transparent dielectric layer; a first partially reflective and partially transmissive metal layer disposed over the first transparent dielectric layer; a second transparent dielectric layer disposed over the first metal layer; a second partially reflective and partially transmissive metal layer disposed over the second transparent dielectric layer; and a third transparent dielectric layer disposed over the second metal layer, wherein the optical structure is configured to produce a first color in reflection mode and a second color different from the first color in transmission mode, wherein the optical structure is configured as a pigment, a paint, an ink, or a foil.
 2. The optical structure of claim 1, wherein the second transparent dielectric layer has a refractive index less than 1.65.
 3. The optical structure of claim 1, wherein the second transparent dielectric layer has a refractive index greater than or equal to 1.65.
 4. The optical structure of claim 1, having a transmission peak comprising: a maximum transmittance; and a spectral bandwidth defined by a full width of the transmission peak at 50% of the maximum transmittance, wherein the maximum transmittance is at least 50%, and wherein the spectral bandwidth of the transmission peak is greater than 2 nm.
 5. The optical structure of claim 4, wherein the spectral bandwidth of the transmission peak is greater than or equal to about 10 nm and less than or equal to about 200 nm.
 6. The optical structure of claim 4, wherein the maximum transmittance is at a wavelength between about 400 nm and about 700 nm.
 7. The optical structure of claim 4, further comprising a reflection peak comprising: a maximum reflectance; and a spectral bandwidth defined by a full width of the reflection peak at 50% of the maximum reflectance, wherein the maximum reflectance is at least 50%, and wherein the spectral bandwidth of the reflection peak is greater than 2 nm.
 8. The optical structure of claim 7, wherein the spectral bandwidth of the reflection peak is greater than or equal to about 10 nm and less than or equal to about 200 nm.
 9. The optical structure of claim 7, wherein the maximum reflectance is at a wavelength between about 400 nm and about 700 nm.
 10. The optical structure of claim 7, wherein the maximum transmittance is at a first wavelength, and wherein the maximum reflectance is at a second wavelength different from the first wavelength.
 11. The optical structure of claim 1, configured to display the first color when viewed by an average human eye along a direction normal to a surface of the optical structure in reflection mode and the second color different from the first color when viewed by an average human eye along a direction normal to a surface of the optical structure in transmission mode.
 12. The optical structure of claim 11, wherein the first color shifts to a third color when viewed by an average human eye along a direction at an angle away from the normal to the surface of the optical structure in reflection mode.
 13. The optical structure of claim 11, wherein the second color shifts to a fourth color when viewed by an average human eye along a direction at an angle away from the normal to the surface of the optical structure in transmission mode.
 14. The optical structure of claim 1, wherein the first or the second metal layer has a thickness greater than or equal to about 5 nm and less than or equal to about 35 nm.
 15. The optical structure of claim 1, wherein the second transparent dielectric layer has a thickness greater than or equal to about 100 nm and less than or equal to about 2 microns.
 16. The optical structure of claim 1, wherein first or the third transparent dielectric layer has a thickness greater than or equal to about 100 nm and less than or equal to about 500 nm.
 17. The optical structure of claim 1, further comprising an encapsulating layer comprising silica, wherein the encapsulating layer surrounds the first, second, and third dielectric layers and the first and second metal layers.
 18. The optical structure of claim 1, wherein the first or the second metal layer comprises at least one of aluminum, silver, gold, silver alloy, or gold alloy.
 19. The optical structure of claim 1, wherein the second transparent dielectric layer comprises at least one of SiO₂, MgF₂ or a polymer.
 20. The optical structure of claim 1, wherein the first or the third transparent dielectric layer comprises at least one of zinc oxide (ZnO), zinc sulfide (ZnS), zirconium dioxide (ZrO₂), titanium dioxide (TiO₂), tantalum pentoxide (Ta₂O₅), ceric oxide (CeO₂), yttrium oxide (Y₂O₃), indium oxide (In₂O₃), tin oxide (SnO₂), indium tin oxide (ITO), tungsten trioxide (WO₃), or combinations thereof.
 21. The optical structure of claim 1, wherein the first or the second metal layer has a thickness greater than or equal to about 5 nm, or less than or equal to about 35 nm.
 22. The optical structure of claim 1, configured as a pigment, a paint or an ink.
 23. A banknote or a document comprising the optical structure of claim
 1. 24. The banknote or document of claim 23, wherein the optical structure is configured as laminate, a hot stamp, or a label that is attached to the banknote or document, or wherein the optical structure is configured as a security thread that is inserted in the banknote or document, or wherein the banknote or document comprises a window, the optical structure incorporated in the window.
 25. The optical structure of claim 11, wherein the first color and the second color do not exhibit color shift when viewed by an average human eye along a direction at an angle away from the normal to the surface of the optical structure in reflection mode or transmission mode. 